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Behavior  of  Phosphoric  M 


A  THES[\\ 


SUBMITTED  TO  THE  UNIVERSITv    i  1.  i,  L_  .    ui 
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Behavior  of  Phosphoric  Acid  in  the  vSoil 


A  THESIS. 


SUBMITTED  TO  THE  UNIVERSITY  FACULTY  OF  CORNELIy 

UNIVERSITY  FOR  THE  DEGREE  DOCTOR 

OF  PHIIvOSPHY 


BY 


JAMES  ADRIAN  BIZZELL 


1903 


PRESS   OF 

THE    ITHACA    JOURNAL 

ITHACA,    N.    Y. 


The  author  takes  this  occasion  to  acknowledge  the  kindness 
of  Professors  G.  C.  Caldwell,  L.  M.  Dennis  and  E.  M.  Chamot, 
of  the  Department  of  Chemistry,  under  whose  guidance  the  fol- 
lowing work  was  carried  out. 


CONTENTS 


Historical 5 

Summary 20 

Experimental 

I.     Methods  of  Determining  Phosphoric  Acid 22 

II.     Relation  between  Phosphoric  Acid  and  Compounds 

of  Iron  and  Calcium 23 

(a)  action  of  lime  on  ferric  phosphate 24 

(b)  action  of  lime  on  the  phosphate  formed 

by  fixation 27 

(c)  action  of  lime  on  soils 31 

(d)  fixation   of    phosphoric    acid   in   the 

presence  of  compounds  of  iron  and 

calcium 34 

III.  Behavior  of  Phosphoric  Acid  towards  Humus 35 

(a)  absorption 

(b)  solution 40 

IV.  Absorption  of  Phosphoric  Acid  by  Zeolites 41 

V.     Action  of  Ferrous  Sulphate 44 

Conclusions 46 


239388 


HISTORICAL 


The  fact  that  soils  are  capable  of  absorbing  certain  substances" 
from  solution  has  been  known  for  nearly  a  century.  There  has 
been  much  discussion  as  to  whom  the  credit  for  this  discovery  is- 
due.  The  first  authentic  record  appears  to  have  been  made  by 
Gazzeri  in  1819.  He  says  :  "If  extract  of  dung  strongly  colored 
and  containing  nutritive  matter  is  added  to  a  clayey  soil,  the  liquid: 
is  rapidly  decolorized.  The  soil  takes  hold  of  the  substances  in 
solution,  and  forms  with  them  compounds  which  are  insoluble,, 
but  which  are  decomposed  by  the  absorbing  action  of  plants." 

Bronner  in  his  treatise  on  "Grape  Culture  in  South  Ger- 
many" published  in  1836,  made  similar  statements  regarding  the 
action  of  sand  and  garden  soil,  and  HuxtableMn  1848,  apparently 
ignorant  of  previous  experiments,  repeated  the  work  of  Gazzeri.. 
A  little  later  the  work  of  these  observers  was  extended  somewhat 
by  Thomson^  of  England. 

In  1850  Way^  made  a  systematic  investigation  of  the  absorb- 
ing power  of  soils.  His  efforts  were  directed  almost  exclusively 
toward  ascertaining  the  cause  and  nature  of  the  absorption  of 
bases.  He  published  only  one  experiment  on  the  absorption  of 
phosphoric  acid.  When  a  solution  containing  calcium  acid  phos- 
phate was  filtered  through  a  soil,  no  trace  of  phosphoric  acid  was 
found  in  the  filtrate. 

The  next  work  bearing  on  the  subject  was  that  of  Wicke^ 
who  found  that  pure  marble  kept  in  contact  with  a  solution  of 
superphosphate  evolved  carbon  dioxide,  and  the  greater  part  of 
the  phosphoric  acid  was  precipitated. 

Thenard^  was  the  first  to  make  any  systematic  study  of  the 
absorption  of  phosphoric  acid  by  soils.  He  succeeded  in  decol- 
orizing the  liquid  from  barnyard  manure  by  the  use  of  iron  and 
aluminum  hydroxides,   and  calcium  carbonate.     He  concluded 


1  Jour.  Roy.  Agr.  See,  England. 

2  Ibid.,  1850,  II,  68. 

3  Ibid.,  1850,  11,313. 

4  Ann.  Chem.  Phar.,  1856,  99,  97. 

5  Compt.  rend.,  44,  819. 


3         • 


that  the  active  constituents  of  manure  are  removed  in  the  soil  by 
these  compounds,  and  that  the  new  compounds  are  decomposed 
slowly  for  the  use  of  the  plant.  In  a  subsequent  investigation^ 
he  always  found  phosphoric  acid  combined  with  aluminum  and 
iron,  and  never  with  calcium  and  magnesium.  He  added  solu- 
tions of  calcium  phosphate  to  the  pure  hydroxides  of  aluminum 
and  iron,  and  to  soil,  and  found  after  a  few  days  that  the  filtered 
water  contained  calcium  but  no  trace  of  phosphoric  acid. 

Deherain"''  examined  a  large  number  of  soils.  Some  of  these 
contained  phosphoric  acid  insoluble  in  water  charged  with  carbon 
dioxide,  while  others  gave  up  phosphoric  acid  to  this  solvent. 
Soils  containing  compounds  insoluble  in  carbon  dioxide  water 
gave  up  phosphoric  acid  to  water  containing  calcium  carbonate 
ammonium  carbonate.  A  solution  of  calcium  carbonate  in  water 
charged  with  carbon  dioxide  acted  on  iron  phosphate  producing 
calcium  phosphate. 

The  role  of  phosphoric  acid  in  the  soil  was  taken  up  by 
Iviebig^  in  connection  with  his  extended  investigations  on  soil 
absorption.  He  determined  the  solvent  effect  of  sodium  chloride, 
sodium  nitrate,  and  ammonium  sulphate  on  phosphates  of  calcium 
and  magnesium.  In  all  cases  appreciable  quantities  of  the  phos- 
phates were  dissolved.  When  the  solutions  thus  made  were 
added  to  soil  the  phosphoric  acid  was  absorbed.  Such  changes 
were  thought  to  have  a  favorable  influence  in  bringing  about  the 
distribution  of  phosphoric  acid  in  the  soil. 

Knop*  confirmed  the  observations  of  Thenard  regarding  the 
role  of  iron  and  aluminum  hydroxides  in  soil  absorption.  He 
showed  that  phosphoric  acid  was  fixed  in  greater  amount  and 
more  rapidly  when  these  compounds  were  added  to  soils,  and 
that  soils  containing  large  quantities  had  greater  reverting  prop- 
erties than  those  containing  small  amounts.  Data  were  not 
accessible. 

Peters^  endeavored  to  ascertain  the  state  in  which  phosphoric 
acid  is  held   in  soils.     He  manured  a  soil  with  bone  dust  and 


1  Compt.  rend.,  46,  212. 

2  Ibid.,  47,  47,  988. 

3  Am.  Chem.  Phar.,  106,  185. 

4  Jahresbericht,  1865,  804. 

5  Ann.  Landw.,  1867,  31. 


treated  the  mixture  with  different  solvents.  From  his  results,  lie 
-concluded  that  phosphoric  acid  in  soils  is  almost  entirely  com- 
bined with  iron  and  aluminum,  and  that  from  a  solution  of  cal- 
•cium  phosphate  in  carbon  dioxide  water,  phosphoric  acid  is  re- 
moved by  soils  only  when  then  the  latter  contain  hydroxides  of 
iron  and  aluminum.  Soils  deprived  of  these  compounds  by  treat- 
ment with  acids  are  apparently  indifferent  to  the  phosphate 
solution. 

In  studying  the  fixing  power  of  soils  for  phosphoric  acid 
Voelcker^  placed  weighed  quanties  of  six  different  kinds  of  loams 
in  bottles,  and  added  known  quantities  of  superphosphate  solu- 
tion. He  noted  from  time  to  time  the  amount  of  phosphoric  acid 
still  remaining  in  solution.  The  hydroxides  present  in  the  soils 
varied  from  zero  to  17.38  per  cent,  and  the  carbonate  of  calcium 
from  0.15  per  cent  to  67.5  per  cent.  The  phosphoric  acid  was 
fixed  with  especial  ease  by  those  soils  which  contained  a  good 
store  of  calcareous  matter.  The  power  of  the  clay  soils  to  render 
the  soluble  phosphoric  acid  insoluble  was  far  less  than  that  of 
chalky  soils. 

Warrington^  determined  the  effect  of  several  substances  on 
the  solubility  of  freshly  precipitated  tri-calcium  phosphate.  He 
found  that  one  per  cent  solutions  of  ammonium  chloride  increavSed 
the  solubility  in  pure  water,  but  that  its  addition  to  water  sat- 
urated with  carbon  dioxide  produced  practically  no  effect.  He 
observed  also  that  when  an  excess  of  calcium  carbonate  was  pres- 
ent, the  amount  of  tri-calciura  phosphate  dissolved  by  carbon 
dioxide  water  was  excessively  small.  The  water  became  satu- 
rated with  calcium  carbonate,  while  only  trace  of  the  phosphate 
•entered  into  solution.  Even  a  very  small  amount  of  calcium 
carbonate  was  capable  of  producing  this  effect. 

Two  years  later^  the  same  investigator  carried  out  an  elab- 
orate experiment  to  show  that  the  absorptive  action  of  soil  towards 
phosphoric  acid  is  due  to  the  formation  of  insoluble  phosphates, 
by  combination  with  hydroxides  of  iron  and  aluminum.  Soils 
freed  from  lime  were  treated  with  solutions  of  calcium  phosphate 


1  Jour.  Roy.  Agr.  Soc,  1863,  24,  46. 

2  Jour.  Chem.  Soc,  1866,  19,  296. 

3  Ibid.,  1868,  21,  I. 


8 

in  carbon  dioxide  water.  Almost  all  the  phosphoric  acid  was  re- 
tained by  the  soil,  while  the  greater  part  of  the  calcium  went  into 
solution.  The  same  results  were  obtained  when  hydroxides  of 
iron  and  aluminum  were  used  instead  of  soil. 

Beyer  and  Biedermann^  worked  on  the  absorption  of  phos- 
phoric acid  by  soils.  Using  a  solution  of  sodium  phosphate  they 
found  no  definite  relation  between  the  amount  absorbed  and  the 
quantities  of  iron,  aluminum,  and  calcium  contained  in  the  soil, 
though  there  was  increased  absorption  with  increased  quantities 
of  these  constituents.  They  determined  the  solvent  action  of 
solutions  of  calcium  sulphate,  sodium  chloride,  sodium  nitrate, 
potassium  chloride,  and  ammonium  sulphate  on  the  compound 
formed  by  absorption.     No  appreciable  effect  was  observed. 

Trochot^  in  his  observations  on  volcanic  soils  noticed  that 
fair  amounts  of  lime  and  phosphoric  acid  were  present,  and  that 
the  fertility  of  these  soils  was  very  high  in  spite  of  their  shallow- 
ness. The  conclusion  was  drawn  that  the  lime  rendered  the 
insoluble  phosphates  available. 

The  fact  that  the  phosphoric  acid  of  superphosphate  grad- 
ually becomes  insoluble  in  ammonium  citrate  solution  was 
observed  by  Millott^  and  Joulie*.  Millot  found  the  change 
to  be  due  to  the  presence  of  large  quantities  of  iron  and  alumium 
oxides,  while  the  results  of  Joulie  seem  to  show  that  when  an 
insufficient  quantity  of  sulphuric  acid  to  completely  decompose 
the  phosphate  rock  is  used,  (a  condition  which  obtains  in  actual 
practice)  the  change  is  due  to  the  formation  of  tri-calcium  phos- 
phate. 

These  experiments  were  extended  somewhat  by  Albert  and 
Vollbrecht^  They  applied  superphosphate  to  some  soils  con- 
taining large  amounts  of  lime,  and  to  others  containing  small 
amounts.  With  soils  containing  small  quantities  of  lime,  6.5 
per  cent  of  the  phosphoric  acid  of  the  superphosphate  had  become 
insoluble  in  ammonium  citrate  after  eight  days,  while  w4th  those 
rich  in  lime,  15.4  per  cent  of  the  phosphoric  acid  had  become 


1  Chem.  Centr.,  1869,  945. 

2  Compt.  rend.,  81,  1027. 

3  Bied.  Centr.,  1875,  89—1879,  408. 

4  Ann.  Chim.  Phys.,  Series  5,  1879,  244. 

5  Bied.  Centr.,  1880,  84. 


insoluble.  These  results  were  construed  as  proving  the  rever- 
sion of  phosphoric  acid  by  soils  to  insoluble  phosphate  of  lime. 
In  another  experiment  precipitated  phosphates  of  iron  and 
aluminum  were  added  to  soils  with  the  result  that  none  of  the 
phosphoric  acid  became  insoluble  in  ammonium  citrate  during 
eight  days. 

Petermann^  attempted  to  determine  the  crop  producing 
power  of  these  various  forms  of  phosphoric  acid.  The  "citrate 
soluble"  was  in  many  cases  superior  to  the  "water  soluble," 
though  the  results  were  not  altogether  satisfactory.  The  low 
value  of  the  water  soluble  was  due  apparently  to  leaching,  while 
the  citrate  soluble,  being  insoluble  in  water,  was  retained  for  the 
use  of  the  plant. 

Solubility  determinations  of  the  compounds  supposed  to 
exist  in  superphosphate  were  made  by  Albert  and  Wagner^  Di- 
calcium  phosphate  was  found  to  be  soluble  in  water  charged  with 
carbon  dioxide.  Sulphates  and  chlorides  appeared  to  have  very 
little  solvent  action,  while  nitrates  and  carbonates  were  more 
effective.  The  solubility  of  precipitated  phosphates  of  iron  and 
aluminum  in  carbon  dioxide  water  was  a  great  deal  less  than 
that  of  precipitated  calcium  phosphate.  Drying  these  salts  les- 
sened their  solubility.  Regarding  the  absorption  of  phosphoric 
acid  by  soils  they  found  a  solution  of  the  phosphates  in  carbon 
dioxide  water  similar  in  its  action  to  a  solution  of  "soluble  phos- 
phoric acid. ' '  Absorption  was  most  complete  in  clays  poor  in 
lime,  less  complete  in  calcareous  clays  and  organic  soils,  and 
very  small  in  sands  poor  in  lime.  An  increase  of  humus  in  the 
soils  rich  in  lime  and  clay  was  always  accompanied  by  an  increase 
in  absorptive  power. 

In  contradiction  to  the  work  just  quoted,  Fiedler^  states  that 
solutions  of  nitrates  dissolve  less  phosphoric  acid  from  soils  than 
pure  water.  His  results  showed  absorption  to  be  favored  silghtly 
by  sodium  nitrate.  A  solution  of  sodium  nitrate  did  not  affect 
the  solubility  of  tri-calcium  phosphate  but  it  acted  on  di-calcium 
phosphate  and  phosphates  of  iron  and  aluminum. 


1  Bied.  Centr.,  1880,  87. 

2  Ibid.,  1880,  640. 

3  I^andw.  Versuchs-Stat.,  26,  135. 


lO 


Kostitcheff^  working  with  pure  compounds,  obtained  a  de- 
composition of  calcium  carbonate  in  the  presence  of  water  and 
phosphates  of  iron  and  aluminum.  Carbon  dioxide  was  evolved 
and  calcium  phosphate  was  formed.  The  change  took  place  even 
when  there  was  present  a  large  excess  of  iron  with  respect  to  the 
phosphoric  acid. 

Tuxen^  investigated  two  soils — a  sand  and  a  clay — in  order 
to  ascertain  the  effect  of  chlorides  of  sodium  and  potassium,  and 
sodium  nitrate  on  the  absorption  of  plant  food.  Phosphoric  acid 
was  readily  taken  up  in  the  presence  of  the  alkalies.  Potassium 
chloride  was  the  most  effective,  causing  an  increase  in  absorption 
of  from  25  per  cent  to  40  per  cent.  Solutions  of  these  salts  were 
found  to  have  little  effect  on  the  solubility  of  soil  phosphoric  acid. 

Krocker  and  Grahl^  carried  on  manuring  experiments  with 
phosphates  of  various  kinds.  Oats  and  beets  seemed  to  be  little 
benefited  by  the  soluble  phosphates  when  used  alone.  When  used 
in  conjunction  with  ammonium  sulphate  there  was  decided  in- 
crease in  the  crop,  the  greatest  yield  being  obtained  by  the  use  of 
bone  meal. 

Similar  experiments  were  made  by  Hoffmeister*.  The  vari- 
ous phosphates  were  applied  as  a  top  dressing.  The  results  are 
interesting.  In  a  sandy  soil  poor  in  lime  the  soluble  phosphoric 
acid  did  not  descend  more  than  ten  inches.  Di-calcium  phos- 
phate remained  unaltered  during  the  experiment,  and  the  mono- 
calcium  phosphate  was  not  converted  into  the  tri-basic  form. 

Fleischer  and  Kissling^  studied  the  influence  exerted  by  cer- 
tain salts  on  the  solubility  of  the  phosphates  present  in  peaty 
soils.  Among  the  salts  used  were  the  sulphate,  chloride,  and 
carbonate  of  potassium,,  the  sulphate  and  chloride  of  calcium, 
sodium  nitrate,  ammonium  sulphate,  and  kainite.  Potassium 
Sulphate  was  very  effective,  while  sodium  nitrate  and  kainite 
were  far  less  so. 

Gladding^  claims  to  have  been  the  first  to  prove  by  direct 


1  Bull.  Soc.  Chim.,  (2)  1880,  34,  341. 

2  r,andw.  Versuchs  Stat. ,  27,  107. 

3  Bied.  Centr.,  1882,  1154, 

4  Ibid.,  1881,  813. 

5  Ibid.,  1883,  153. 

6  Chem.  News,  50,  16,  27. 


II 


laboratory  experiment,  that  the  greater  part  of  the  phosphoric 
acid  reverted  in  soils  is  in  the  form  of  iron  and  aluminum  phos- 
phates. On  adding  a  solution  of  superphosphate  to  three  repre- 
sentative soils  he  found  after  several  days  that  all  the  phosphoric 
acid  was  soluble  in  ammonium  citrate  at  a  temperature  of  65 
degrees  C.  "This"  the  author  states  "shows  that  most  of  the 
phosphoric  acid  was  present  in  the  form  of  iron  and  aluminum 
phosphates,  since  tri-calcium  phosphate  is  much  less  readily  at- 
tacked by  this  reagent  than  these  phosphates."  The  author 
criticizes  results  obtained  by  Albert  and  Vollbrecht\  from  which 
these  investigators  concluded  that  the  phosphoric  acid  not  dis- 
solved by  ammonium  citrate  at  40  degrees  C  had  become  insoluble 
tri-calcium  phosphate.  Unfortunately  Gladding  did  not  give  the 
composition  of  the  soils  used  in  his  experiments. 

The  composition  and  action  of  superphosphate  have  been 
studied  from  many  points  of  view.  Weilandt^  found  it  to  be  very 
completely  absorbed  by  marl.  The  action  was  rapid.  Marge- 
stein^  showed  its  value  to  be  increased  by  mixing  with  wood 
ashes.  He  grew  potatoes,  mustard,  barley,  and  maize  on  a 
diluvial  sandy  soil.  Quantities  of  ashes  up  to  25  per  cent  were 
beneficial.     Analyses  of  soils  UvSed  were  not  given. 

From  his  results  Joffre*  referred  reversion  to  the  formation 
of  phosphate  of  iron.  He  determined  its  coefficient  of  solubility 
in  water,  in  water  charged  with  carbon  dioxide,  and  in  solutions 
of  diiFerent  salts.  In  all  cases  it  was  less  sensitive  to  these 
substances  than  tri-calcium  phosphate.  Vegetation  tests  showed 
it  to  be  less  valuable. 

Thompson^  determined  the  absorptive  power  of  sand,  as  well 
as  that  of  the  same  sand  containing  known  quantities  of  ortho- 
clase,  calcium  carbonate,  iron  and  aluminum  hydroxides,  of 
mixtures  of  calcium  carbonate  and  orthoclase,  and  of  calcium 
carbonate  and  hydroxides  of  iron  and  aluminum.  He  also 
noticed  the  effect  of  sodium  chloride  and  potassium  nitrate  on 
the  process  of  absorption.      The  original  publication   was  not 


1  Bied.  Centr.  1880,  87. 

2  I^andw.  Versuchs-Stat.,  34,  207. 

3  Bied.  Centr.,  1888,  225. 

4  Bull.  Soc.  Chim.,  47,  312. 

5  Inaug.  Dissertation,  Dorpat,  1890. 


12 

-accessible  but  abstracts  give  the  following  conclusions.  '  'Sand 
offers  no  resistance  to  the  extraction  of  the  phosphoric  of  super- 
phosphate by  water.  Addition  of  orthoclase  produces  no  effect. 
Calcium  carbonate  combines  quickl}^  with  soluble  phosphoric 
acid.  Iron  and  aluminum  hydroxides  are  also  active  in  retain- 
ing phosphoric  acid.  The  compounds  formed  with  iron  and 
aluminum  are  more  stable  towards  salt  solutions  than  tri-calcium 
phosphate.  Sodium  chloride  solutions  (i  per  cent  and  2  per 
cent)  dissolve  less  phosphoric  acid  from  superphosphate  than 
pure  water.  In  the  presence  of  calcium  carbonate  and  hydroxi- 
des of  iron  and  alumium  the  salt  solutions  dissolve  more  than 
pure  water. ' ' 

The  action  of  carbon  dioxide  on  tri-calcium  phosphate  alone 
and  in  the  presence  of  ferric  hydroxide  was  studied  by  Geogievice\ 
Tri-calcium  phosphate  suspended  in  water  was  decomposed  by 
carbon  dioxide,  but  the  reaction  was  far  from  being  complete. 
When  ferric  hydroxide  was  also  present  decomposition  took  place, 
but  the  phosphoric  acid  immediately  combined  with  the  iron. 
Under  some  conditions  the  whole  of  the  phosphoric  acid  was  with- 
drawn from  the  calcium  salt.  These  results  confirm  the  observa- 
tions of  many  previous  workers,  namely,  that  all  of  the  phosphoric 
acid  in  calcium  phosphate  when  applied  to  the  soil  finally  becomes 
converted  to  phosphate  of  iron. 

Some  interesting  facts  regarding  the  assimilation  of  phos- 
phoric acid  by  crops  were  obtained  by  Deherain^  on  the  experi- 
mental plots  at  Grignon.  He  noticed  that  the  sum  of  the  phos- 
phoric acid  in  drainage  and  crops  in  ten  years,  was  not  equal 
to  the  phosphoric  acid  not  extracted  by  acetic  acid.  He  con- 
cluded that  the  calcium  phosphate  was  changed  to  iron  and 
aluminum  phosphates  after  a  time.  A  soil  was  mixed  with  tri- 
calcium  phosphate  and  placed  in  a  seltzogene.  After  a  few  days 
no  phosphoric  acid  was  dissolved  by  water  charged  with  carbon 
•dioxide,  although  tri-calcium  phosphate  is  soluble  in  this  mix- 
ture. He  does  not  state  whether  the  soil  contained  lime.  Ferric 
and  aluminum  phosphates  were  assimilated  by  oats  but  not  by 
wheat. 


1  Monatsh.,  1891,  12,566. 

2  Ann.  Agron.,  17,  445. 


13 

Kellner^  made  some  observations  on  the  action  of  lime  as-^- 
manure  on  paddy  fields.  Complete  infertility  had  resulted  in 
several  places.  He  attributed  part  of  this  to  loss  by  leaching. 
Among  other  things  the  action  of  phosphates  in  two  soils  was  in- 
vestigated. He  mixed  in  from  0.25  percent  to  5  per  cent  of 
quicklime,  and  after  two  weeks  added  a  solution  of  potassium  tri- 
hydrogen  phosphate.  Lime  apparently  caused  an  increase  in  the 
soluble  phosphoric  acid.  The  maximum  effect  was  obtained  with 
I  per  cent  to  2.5  lime.  After  two  months  still  more  phosphoric 
acid  had  become  soluble. 

Gerlach"'^  repeated  Thompson's  work^on  the  absorption  of 
phosphoric  acid  by  soils.  The  results  here  recorded  show  that 
clay,  peat,  and  sand  which  have  been  extracted  with  hydrochloric 
acid  have  no  power  of  absorbing  free  phosphoric  acid,  sodium 
phosphate,  and  superphosphate.  Calcium  and  magnesium  car- 
bonates absorb  phosphoric  acid,  while  iron  and  aluminum  oxides 
do  so  very  completely.  In  the  first  case  the  compounds  formed 
are  comparatively  soluble  and  are  completely  extracted  by  water 
charged  with  carbon  dioxide.  The  compounds  formed  with  iron 
and  aluminum  are  insoluble  in  water  even  in  the  presence  of  car- 
bon dioxide,  but  are  more  or  less  dissolved  by  the  prolonged 
action  of  organic  acids.  He  endeavored  to  determine  the  form 
in  which  phosphoric  acid  is  absorbed.  A  solution  of  mono-cal- 
cium phosphate  was  placed  in  contact  with  calcium  carbonate. 
Di-  and  tri-calcium  phosphates  were  formed  depending  on  the 
relative  amounts  of  the  materials  used.  When  iron  and  aluminum 
hydroxides  were  the  absorbents,  not  only  was  the  phosphoric  acid 
absorbed,  but  the  calcium  was  also  retained.  Di-calcium  phos- 
phate and  an  iron  phosphate  were  formed.  The  author  concluded 
that  normal  ferric  phosphate  is  formed  when  sufficient  ferric 
hydroxide  is  present.  A  basic  phosphate  of  iron  was  also  con- 
sidered a  possibility. 

Stocklasa''  estimated  the  yearly  loss  of  calcium  carbonate 
from  various  soils.     The  results  are  of  interest  as  bearing  on  the 


1  Bull.  Coll.  Agr.  Tokyo  Imp.  Univ.,  1891 

2  I^andw.  Versuchs-Stat.,  1895,  46.  201. 

3  Iiiaug.  Dissertation,  Dorpat.,  1890. 

4  Bied.  Centr.,  1895,  82. 


14 

loss  of  phosphoric  acid.  Four  soils — loam,  marl,  clay  and  humus 
— were  anal5^sed  and  the  total  phosphoric  acid  compared  with 
that  found  in  the  drainage  water.  From  the  data  thus  obtained 
the  annual  loss  was  estimated.  The  results  showed  a  loss  of 
9146  grams  per  hectare  in  the  case  of  the  clay,  while  the  humus 
lost  21995  grams  per  hectare.  The  humus  contained  the  smallest 
per  cent  of  phosphoric  acid,  but  lost  the  largest  amount. 

Schreiber^  grew  oats  followed  by  turnips  on  sandy,  humus, 
and  loamy  soils  using  two  fertilizers,  viz  :  (i)  a  mixture  of  di- 
calcium  phosphate,  calcium  sulphate,  and  magnesium  carbonate, 
(2)  a  mixture  of  sodium  phosphate,  and  carbonates  of  calcium 
and  magnesium.  The  first  mixture  gave  the  best  results  in 
every  case.  The  author  thought  that  the  diminished  action  in 
the  second  case  was  due  to  the  precipitation  of  phosphoric  acid 
by  the  carbonates  of  calcium  and  magnesium.  His  results  with 
peaty  soils  showed  that  the  humus  phosphoric  acid,  soluble  in 
alkaline  ammonium  citrate,  was  almost  useless  for  vegetation. 
In  some  cases  the  humus  acted  on  the  assimilable  phosphoric 
acid  in  a  manner  analagous  to  calcium  carbonate. 

JofTre^  made  vegetation  experiments  to  determine  the  relative 
values  of  superphosphate,  tri-calcium  phosphate,  and  the  ferric 
phosphate  obtained  from  a  sample  of  manure  containing  very 
little  tri-calcium  phosphate.  Crops  of  mustard  were  grown  on 
plots  which  differed  only  with  regard  to  the  nature  of  the  phos- 
phate present.  The  ferric  phosphate  was  little  better  than  no 
fertilizer,  while  the  other  phosphates  gave  a  large  increase. 

On  mixing  basic  slag  and  superphosphate  with  soil,  Smora- 
wski  and  Jacobson'*  noticed  that  the  phosphoric  acid  originally 
soluble  in  water  was  rapidly  converted  to  the  citrate  soluble 
form,  and  this  compound  underwent  no  further  change. 

Stoklasa'*  called  attention  to  the  fact  that  ferrous  salts  in  the 
presence  of  phosphoric  acid  soluble  in  water,  give  rise  to  the 
production  of  di-tri-ferric  phosphate,  unless  there  is  an  excess  of 
free  phosphoric  acid   present.       With    soluble   phosphoric  acid 


1  Exp.  Sta.  Rec,  1B95,  804. 

2  Bull.  Soc.  Chim.,  1896,  (3)  15,  42. 

3  Bied.  Centr.,  1896,  580. 

4  Ann.  Agron.,  1897,  23,  588. 


15 

aluminum  salts  do  not  form  compounds  analagous  to  those  o^ 
ferrous  and  ferric   salts,   but   behave   like  salts  of  calcium  and 
magnesium.     No  data  were  given. 

Prianischnikoff ^  working  on  the  relative  values  of  mineral 
phosphates,  made  sand  culture  experiments  with  cereals  manured 
with  phosphorites.  The  phosphoric  acid  was  only  slightly  avail- 
able. Under  the  same  conditions  certain  other  plants  such  as 
peas,  lupines,  buckwheat,  and  mustard  seemed  to  be  able  to  util- 
ize the  phosphoric  acid.  The  "podzols"  (soils  containing  a 
large  amount  of  fine  siliceous  and  organic  matter)  were  appar- 
ently able  to  render  phosphorites  available  for  cereals  and  other 
plants.  The  same  was  true  of  forest  and  peaty  soils.  Some  of 
the  black  soils,  however,  were  without  action  on  the  phos- 
phorites. 

The  results  of  numerous  experiments  conducted  by  Ullmann 
and  Grimm^  showed  that  for  months  after  the  application  of 
superphosphate,  phosphoric  acid  soluble  in  water  passed  through 
a  depth  of  soil  equal  to  ten  inches.  The  authors  found  only  a 
portion  of  the  magnesia,  lime,  and  oxides  of  iron  and  aluminum, 
available  for  retaining  phosphoric  acid.  The  mechanical  fixa- 
tion appeared  to  depend  on  the  amount  of  fine  sand  present, 
especially  sand  of  the  fineness  of  dust. 

A  series  of  experiments,  on  the  use  of  lime,  extending  over 
several  years,  have  recently  been  carried  out  at  the  Rhode  Island 
Experiment  Station^.  The  practical  results  obtained  in  field 
tests  by  the  use  of  lime  in  conjunction  with  phosphates  are  of 
particular  interest,  and  apparently  contradictory  to  a  majority  of 
the  writers  on  agricultural  chemistry.  Because  of  its  peculiar 
interest,  the  work  is  quoted  here  in  some  detail.  Crops  were 
grown  for  four  successive  years,  and  the  average  yields  on  the 
limed  and  the  unlimed  plots  recorded.  The  results  on  the  yields 
of  hay  are  given  herewith  as  being  typical  of  those  obtained 
with  the  crops  grown.     The  figures  represent  pounds  per  acre. 


1  Ann.  Agron.,  1899,  25,  177. 

2  Chem.  Ind.,  1900,  23,  61. 

3  R.  I.  Agr.  Expt.  Sta.  Bull.,  No.  58. 


i6 

Forms  of  Phosphoric  Acid  Applied.  lyimed.  Unlimed.         Gain  from 

lyiming. 

Dissolved  Boneblack 19,837  9,820  10,016 

Dissolved  Bone 19,281  8,564  10,716 

Dissolved  Phosphate  Rock 20,205  8,951  11.253 

Fine  Ground  Bone 22,012  11,855  10,  ^57 

Basic  Slag  Meal 20,400  13,193  7,206 

Floats 20,525  10,560  9.965 

Alumina  Phosphate  (raw) 14,387  5,042  9,345 

Alumina   Phosphate  (ignited) 19,481  4,93°  I4,55i 

No  Phosphoric  Acid 15, 737  2,547  13,190 

Double  Superphosphate 17, 937  4,752  13,184 

It  will  be  seen  that  there  was  a  wonderful  gain  in  the  crop, 
in  all  cases,  resulting  from  the  use  of  lime.  The  acidulated  phos- 
phates appeared  to  be  helped  more  by  the  lime  than  the  unacidu- 
lated  forms.  The  authors  suggested  that  lime  might  have  been 
beneficial  by  correcting  the  acidity  of  the  first.  No  experiments, 
however,  were  made  to  ascertain  whether  the  lime  would  liberate 
phosphoric  acid  from  the  insoluble  phosphates  of  the  soil. 

Some  rather  peculiar  results  were  obtained  by  Schreiber^  on 
the  action  of  calcium  carbonate  on  mineral  phosphates.  Addi- 
tion of  calcium  carbonate  to  the  phosphates  decreased  the  action 
of  the  latter  on  the  crops  grown,  the  greater  the  amount,  the 
greater  the  injury.  In  some  cases  the  effect  extended  to  the 
next  year.     The  action  of  basic  slag  was  not  affected. 

Kellner  and  Bottcher'"*  obtained  similar  results,  when  using 
superphosphate,  basic  slag,  and  bone  meal  for  oats.  Addition  of 
calcium  carbonate  caused  reduced  yields  in  every  case,  but  this 
decrease  was  less  with  the  superphosphate  and  slag  than  with 
the  bone  meal.  Comparing  the  amounts  of  increase  due  to  phos- 
phoric acid  applied,  the  addition  of  calcium  carbonate  was  not 
unfavorable  when  used  in  conjunction  with  slag  and  super- 
phosphate. With  bone  meal  the  addition  of  calcium  carbonate 
reduced  the  yields  on  the  average  by  67  per  cent.  No  explana- 
tion was  offered  for  the  action  of  the  latter. 

The  relation  between  humus  and  the  phosphoric  acid  of  the 
^oil,  has  not  received  very  much  attention.     Incidentally,  obser- 

1  Bied.  Centf.,  igoo,  162. 

2  Deut.  I,audw.  Presse,  1900,  27,  665. 


17 

vatious  have  been  made  on  the  retentive  and  solvent  powers  of 
some  organic  soils,  but  in  most  cases  the  work  was  designed  for 
the  investigation  of  other  problems. 

Fleischer  and  Kissling\  in  a  study  of  the  action  of  moorland 
soils  on  insoluble  phosphates,  found  that  the  effect  was  to  render 
a  portion  of  the  phosphate  soluble  in  water,  amounting  in  one 
case  to  5.5  per  cent  of  the  total.  At  the  same  time  a  portion  was 
reduced  to  the  di-calcium  salt,  and  in  one  compost  as  much  as  17 
per  cent  of  the  total  was  brought  into  this  form.  When  the  ratio 
of  soil  to  phosphate  was  widened,  there  was  an  increase  in  soluble 
salt.  This  increase  varied  directly  with  the  time  of  contact. 
There  appeared  to  be  a  limit  beyond  which  the  soluble  salt  became 
reduced. 

In  1892  Berthelot  and  Andre^  made  some  experiments  on  the 
absorption  of  phosphoric  acid  by  artificial  humic  anhydride  pre- 
pared from  sugar.  Solutions  of  phosphates  of  sodium  and  am- 
monium were  mixed  with  the  acid  anhydride  and  allowed  to 
stand  in  the  cold  for  twenty-four  hours.  The  absorption  from 
these  salts  were  very  small.  In  most  cases  practically  no  phos- 
phoric acid  was  retained. 

The  action  of  humic  acid  in  the  soils  on  the  solubility  of 
various  natural  phosphates  was  taken  up  by  Minssen  and  Tacked 
Many  of  the  insoluble  phosphates  were  acted  on  in  the  presence 
of  the  free  humic  acid  of  the  soil.  When  lime  was  added  the 
acid  was  neutralized  and  the  solvent  power  thereby  largely 
destroyed.  The  quantity  of  phosphoric  acid  dissolved  increased 
with  an  increase  in  the  quantity  of  free  humic  acid  present.  The 
numerical  agreement,  however,  was  affected  by  the  reabsorption 
of  the  phosphoric  acid  made  soluble. 

Snyder*  prepared  humus  by  mixing  soil  with  such  sub- 
stances as  clover,  flour,  straw,  saw  dust,  and  sugar,  and  allowing 
to  ferment  for  a  year.  At  the  end  of  that  time  the  humus  pro- 
duced contained  more  phosphoric  acid  than  was  originally  present 
in  the  humus- forming  material,  indicating  that  some  of  the  phos- 
phoric acid  of  the  soil  had  united  with  the  humus.     By  *  'humus'^ 


1  Bied.  Centr.,  1883,  155. 

2  Ann.  Chim.  Phys.,  27,  196. 

3  L,andw.  Jahrb.,  1898,  IV,  392. 

4  Minn.  Agr.  Expt.  Sta.  Bull.,  No.  53. 


i8 

is  here  meant  the  soluble  organic  matter  extracted  with  a  2  per 
cent  solution  of  ammonia,  after  treating  the  soil  with  a  dilute 
solution  of  hydrochloric  acid.  The  "humic  phosphoric  acid"  is 
the  phosphoric  acid  present  in  the  extractive  material.  Humic 
phosphoric  acid  proved  to  be  one  of  the  most  valuble  forms. 

Dumont^  investigated  the  abvSorbing  power  of  several  organic 
soils  containing  humus  and  lime.  Mono-calcium  phosphate  was 
applied  in  aqueous  solution  before  and  after  calcination  of  the 
soil.  Calcination  seemed  to  decrease  the  immediate,  but  increased 
the  final  absorption  of  phosphoric  acid.  These  results  were  con- 
strued as  proving  the  combination  of  humus  with  phosphoric 
acid.  The  quantities  of  lime  present  appeared  to  have  same 
effect  on  the  absorption,  but  the  relationship  of  lime,  humus,  and 
phosphoric  acid  was  not  well  defined.  Mono-calcium  phosphate 
was  also  applied  to  precipitated  humus.  Absorption  took  place 
in  the  ratio  of  humus  10,  to  P2O5I.  Precipitated  humus  was  not 
defined. 

The  use  of  ferrous  sulphate  in  agriculture,  has  not  been  the 
object  of  much  experimental  enquiry.  Griffiths^  however,  ob- 
tained some  very  interesting  practical  results  b}^  its  use,  and  his 
work  is  taken  up  here  because  of  its  bearing  on  the  behavior  of 
phosphoric  acid  in  the  soil. 

The  material  was  applied  at  the  rate  of  50  lbs.  per  acre. 
Among  the  crops  grown  were  hay,  mangels,  beans,  potatoes,  and 
wheat.  Applied  with  farmyard  manure,  its  action  was  injurious, 
the  poisonous  effect  being  due  to  the  formation  of  ferrous  sul- 
phide. The  crops  in  nearly  all  other  cases  were  increased  largely 
and  the  percentage  content  in  phosphoric  acid  was  increased  in  all 
cases.  The  ferrous  sulphate  was  not  oxidized  rapidly  in  the  soil, 
its  presence  being  proven  six  weeks  after  application.  Ferrous 
sulphate  was  used  in  conjunction  with  superphosphate  in  all  tests. 

The  subject  was  taken  up  a  little  later  by  Delacharlony  and 
Destremx^  They  found  that  the  sulphate  was  productive  of  in- 
creased yields  only  when  the  soil  contained  small  quantities  of 
iron,  the  increase  ceasing  when  the  ferric  oxide  reached  3  per 


1  Compt.  rend.,  1901,  132,  435. 

2  Jour.  Chem.  Soc,  Trans.  1884.  71.— 1886,  114,-1887,  215. 

3  Bied.  Centr.,  1889,  9. 


19 

•cent,  and  was  detrimental  when  4  per  cent  was  reached.  The 
increase  varied  with  different  crops,  from  5  per  cent  to  139  per 
cent. 

Cazeneuve  and  Nicolle^  studied  the  solvent  action  of  ferrous 
sulphate  on  some  of  the  phosphatic  fertilizers  in  common  use, 
namely,  slag,  mineral  phosphate,  bones,  superphosphate,  and  di- 
calcium  phosphate.  After  eight  days  the  first  three  had  suffered 
no  change  in  the  amount  of  phosphoric  acid  soluble  in  acetic  acid 
and  ammonium  citrate.  With  superphosphate  half  of  the  water 
soluble  had  become  reverted.  With  the  di-calcium  phosphate, 
the  phosphoric  acid  soluble  in  acetic  acid  and  ammonium  citrate 
had  increased,  but  the  change  was  not  to  the  water  soluble  form. 

The  action  of  ferrous  sulphate  on  crops  was  investigated  by 
Boiret  and  PatureP.  Seedlings  of  peas  and  oats  were  used  in 
water  culture  experiments.  Even  with  very  dilute  solutions,  the 
plants  were  poisoned,  owing  to  the  formation  of  free  sulphuric 
acid.  Iron  citrate  and  citric  acid  had  the  same  effect.  Tests 
were  then  made  with  soils  containing  known  quantities  of  lime. 
The  sulphate  was  injurious  except  when  there  was  present  a  suf- 
ficient amount  of  lime  to  neutralize  the  free  acid  formed.  Analysis 
of  the  crops  showed  that  ferrous  sulphate  had  no  effect  on  the 
amount  of  phosphoric  acid  in  the  product.  In  these  experiments 
the  sulphate  was  not  used  in  conjunction  with  other  fertilizers,  as 
in  the  work  of  Griffiths  already  quoted. 


1  Chem.  Centr.,  IV,  (2)  1892,  121. 

2  Ann.  Agron.,  18,  418. 


20 


SUMMARY 


So  far  as  the  writer  has  been  able  to  ascertain,  the  literature- 
cited  above  includes  practically  all  the  work  relating  directly  to 
the  subject  under  investigation.  Briefly  stated,  the  following 
conclusions  have  been  drawn  from  the  work  referred  to  : 

I.  When  soluble  phosphates  are  added  to  soils  the  phos- 
phoric acid  is  usually  changed  to  insoluble  forms.  The  action  in 
most  cases  is  due  to  chemical  change,  by  virtue  of  the  presence 
of  compounds  of  iron,  aluminum,  and  calcium,  and  possibly 
magnesium.  These  compounds  are  easily  decomposable  salts 
which  can  be  extracted  with  hydrochloric  acid. 

II.  Hydrated  oxides  of  iron  and  aluminum,  and  carbonate 
of  calcium  unite  with  phosphoric  acid  very  rapidly.  The  phos- 
phates of  iron  and  aluminum  formed  are  from  two  to  five  times  as 
insoluble  in  pure  water  as  calcium  phosphate.  They  are  also  less 
soluble  in  water  containing  carbon  dioxide  and  various  alkali 
salts. 

III.  It  is  the  tendency  of  phosphoric  acid  to  form  the  more 
difficulty  soluble  compounds  in  the  soil,  but  not  all  the  phosphoric 
acid  appears  to  be  so  combined,  nor  is  such  combination  always 
effected  with  with  great  rapidity.  The  complete  fixation  in 
some  cases  does  not  take  place  for  several  months. 

IV.  Mixed  with  calcium  carbonate,  in  the  presence  of  soil 
water,  phosphates  of  iron  and  aluminum  undergo  decomposition 
with  the  formation  of  calcium  phosphate.  With  certain  soils, 
calcium  hydroxide  also  apparently  has  a  solvent  action  on  the 
phosphates  of  iron.  Conditions  affecting  the  equilibrium  between 
these  compounds  have  not  been  determined. 

V.  Absorption  may  be  influenced  to  some  extent  by  the 
presence  of  some  of  the  alkali  salts.  The  effect  is  not  very  well 
defined.  In  some  cases  the  solubility  of  the  soil  phosphoric  acid 
seems  to  be  increased  in  the  presence  of  these  salts. 

VI.  In  some  cases  the  humus  in  soils  has  the  effect  of  ren- 
dering insoluble  phosphates  more  available  to  plants.    The  action. 


21 

seems  to  be  due  to  free  humic  acid,  rather  than  to  salts  of  this 
acid.     This  point,  however,  has  not  been  determined. 

VII.  Humus  is  believed  to  possess  great  absorptive  prop- 
erties though  experimental  evidence  supporting  this  view  is 
exceedingly  limited  and  not  very  conclusive.  The  relation  be- 
tween humus  and  phosphoric  acid  is  not  well  defined. 

VIII.  Ferrous  sulphate  in  conjunction  with  phosphates, 
applied  to  some  soils  is  beneficial  to  crops.  It  seems  to  increase 
the  availability  of  the  phosphoric  acid.  In  large  quantities  on 
acid  soils,  it  acts  as  a  poison  to  plants.  The  injurious  effect  is 
due  in  part  to  the  formation  of  sulphuric  acid.  The  same  effect 
may  be  produced  in  neutral  soils,  and  soils  not  sufficiently  basic 
to  neutralize  all  the  acid  formed. 

IX.  Sand  and  orthoclase  offer  no  resistance  to  the  extrac- 
tion of  soluble  phosphoric  acid  by  pure  water. 


22 


EXPERIMENTAL 


1.     Methods  of  Determining  Phosphoric  Acid. 

The  work  about  to  be  described  necessitated  a  number  of 
phosphoric  acid  determinations.  Hence  it  was  thought  advisible 
in  most  cases  to  employ  the  Optional  Volumetric  Method  of  the 
Association  of  Official  Agricultural  Chemists^  In  order  to  test  its 
accuracy,  this  method  was  compared  with  the  Official  Gravimetric 
Method^  on  a  variety  of  materials  typical  of  those  to  be  con- 
sidered in  the  experiments  following.  The  methods  were  carried 
out  as  follows  : 

Volumetric  Method.  The  sample  was  dissolved  in  from  15 
to  30  c.c.  of  strong  hydrochloric  acid  and  from  3  to  10  c.c.  of  nitric 
acid.  The  solution  was  cooled,  diluted  to  250  c.c. ,  and  an  aliquot 
part  taken  for  the  determination.  This  portion  was  treated  with 
10  c  c.  of  nitric  acid,  nearly  neutralized  with  ammonia,  diluted 
to  about  100  c.c,  and  heated  in  the  water  bath  to  65  degrees  C. 
Molybdic  solution  was  then  added  (the  amount  depending  on  the 
amount  of  PjOg  present)  the  mixture  stirred,  allowed  to  stand 
thirty  minutes,  and  filtered  on  an  asbestos  filter.  After  thor- 
ough washing  with  cold  water,  the  yellow  precipitate  was  re- 
turned to  the  precipitating  beaker,  dissolved  in  standard  potas- 
sium hydroxide,  a  few  drops  of  phenolphthalein  added,  and  the 
excess  of  alkali  titrated  with  standard  nitric  acid.  Each  cubic 
centimeter  of  the  standard  potassium  hydroxide  was  equivalent  to 
I  mg.  PA- 

Gravimetric  Method.  The  sample  was  dissolved  in  from 
15  to  30  c.c.  of  strong  hydrochloric  acid  and  from  3  to  10  c.c.  of 
nitric  acid.  The  solution  was  cooled  diluted  to  250  c.c,  and  an 
aliquot  part  corresponding  to  0.3  gram  taken  for  analyvSis.  The 
solution  was  treated  with  10  c.c.  of  nitric  acid,  nearly  neutralized 
with  ammonia,  diluted  to  about  100  c.c,  and  heated  to  65  degrees 


1  U.  vS.  Dept.  Agr.,  Div.  Cheni.,  Bui.  No.  46,  13  (revised). 

2  Ibid.,  10. 


23 

C  in  a  water  bath.  Molybdic  solution  was  then  added,  the  whole 
digested  for  one  hour  at  65  degrees  C,  filtered  and  washed  thor- 
oughly with  cold  water.  The  precipitate  was  dissolved  on  the 
filter  with  ammonia  and  hot  water,  knd  washed  into  a  beaker  to 
a  bulk  of  not  more  than  100  c.c.  The  solution  was  nearly  neu- 
tralized with  hydrochloric  acid,  cooled,  and  magnesia  mixture 
added,  drop  by  drop,  from  a  burette,  stirring  vigorously.  After 
fifteen  minutes,  30  c.c.  of  ammonia  (0.96  sp.  gr. )  were  added, 
and  the  whole  allowed  to  stand  over  night.  The  precipitate  was 
then  collected  on  a  filter,  washed  with  2.5  per  cent  ammonia 
until  free  from  chlorides,  dried,  ignited  to  whiteness  and 
weighed.  The  quantity  of  P2O5  was  calculated  from  the  weight 
of  the  Mg^Ffi^,  using  the  factor  0.6396.  The  following  results 
were  obtained. 

Material                                                                 Grams  P2O5 

Gravimetric  Volumetric 

"Insoluble"   from  Superphosphate 0.0145  0.0147 

"Insoluble"  from  Superphosphate 0.0146  0.0147 

Ferric  Phosphate 0.1146  0.1137 

Ferric  Phosphate 0.1149  0.1140 

Aluminum    Phosphate 0.1137  o.  1143 

Aluminum   Phosphate 0.1138  0.1146 

Calcium  Phosphate 0.1361  0.1369 

Calcium  Phosphate 0.1367  0.1374 

The  results  by  the  two  methods  agree  closely,  the  difference 
being  within  the  experimental  error  usually  allowed  for  such 
determinations.  With  the  larger  quantities  of  phosphoric  acid, 
the  gravimetric  method  was  used  as  a  check  on  the  volumetric. 
The  results  in  all  cases,  therefore,  are  strictly  comparable. 

II.     Relation  Between  Phosphoric  Add,  and  Compounds  of  Iron, 

and  Calcium. 

Recent  results  obtained  by  the  use  of  lime  in  conjunction 
with  phosphates  of  various  kinds,  have  suggested  new  possibili- 
ties with  regard  to  the  relation  between  phosphates  of  iron  and 
calcium  in  the  soil.  Lime  has  hitherto  been  regarded,  partly  as 
a  direct  plant  food,  partly  as  an  amendment  in  liberating  plant 
food,  especially  potash,  but  principally  as  an  agent  for  improving 


24 

the  physical  condition  of  soils.  With  regard  to  the  action  of  phos- 
phoric acid  in  the  presence  of  calcium  (lime  and  calcium  carbonate) 
and  iron,  the  literature  is  somewhat  contradictory.  The  reaction 
to  some  extent  appears  to  be  a  reversible  one.  The  mass  law, 
which  of  late  years  has  served  to  clear  up  many  unexplained 
phenomena,  undoubtedly  plays  a  very  important  part. 

It  occurred  to  the  author  that  the  beneficial  effect  of  lime 
might  be  due,  to  some  extent,  to  the  chemical  action  of  this  sub- 
stance in  rendering  phosphates  more  available  to  crops,  and  that 
the  behavior  of  phosphoric  acid  in  the  presence  of  compounds  of 
calcium  and  iron,  might  depend  on  the  relative  quantities  of  the 
last  two  present.  The  experiments  designed  to  study  these 
points  are  described  under  the  following  heads  : 

(a)  Action  of  lime  on  ferric  phosphate. 

(b)  '*        "     "       "  the  phosphate  formed  by  fixation. 

(c)  ''•       "     '•      *'  soils. 

(d)  Fixation  of  phosphoric  acid   in   the  presence  of  com- 

pounds of  iron  and  calcium. 

(a)      ACTION   OF   LIME    ON    FERRIC   PHOSPHATE. 

The  object  of  the  experiment  was  to  determine  the  effect  of 
lime  on  the  availability  of  precipitated  and  natural  ferric  phos- 
phates. 

♦  The  solvent  selected  for  this  purpose  was  the  i  per  cent  citric 
acid  solution  proposed  by  Dyer\  for  the  determination  of  avail- 
able plant  food  in  soils.  This  reagent  is  the  only  one  of  the 
many  solvents  proposed  for  determining  available  plant  food,  that 
has  received  any  favor  among  agricultural  chemists.  While  it  is 
still  of  doubtful  efficiency  in  determining  absolute  availability,  in 
determining  relative  values,  for  which  purpose  it  was  used  in  the 
present  work,  it  has  been  of  unquestioned  service.  Dyer  pro- 
posed this  reagent  as  representing  the  average  root  juice  acidity 
of  a  large  number  of  agricultural  plants  examined  by  him. 

Precipitated  ferric  phosphate  was  prepared  by  treating  fer- 
ric chloride  with  di-sodium  phosphate.  The  resulting  precipitate 
was  washed  thoroughly  with  cold  water  and  the  product  air 

I  Jour.  Cheni.  Soc.  Trans.,  1894,  115. 


25 

dried.  Phosphoric  acid  was  determined  by  the  gravimetric 
method,  and  ferric  oxide  by  the  vohimetric  bi-chromate  method. 
The  following  results  were  obtained  : 

Fe,0,    41.53% 

P2O5 38.20% 

H2O      20.08% 

Correcting  for  hygroscopic  moisture,  the  figures  agree  with 
the  formula  FePO,.2H20. 

The  only  natural  ferric  phosphate  that  could  be  obtained 
was  a  sample  of  Dufrenite  containing  16.01  per  cent  of  P2O5. 
This  is  a  basic  ferric  phosphate. 

The  solubility  of  these  materials  was  first  determined  by 
placing  I  gram  in  a  small  flask,  adding  100  c.c.  of  i  per  cent 
citric  acid  solution,  and  allowing  to  stand  twenty-four  hours  at 
room  temperature,  shaking  once  each  hour.  The  insoluble  resi- 
due was  then  filtered  off,  washed  with  cold  water,  and  the  phos- 
phoric acid  determined.     The  following  results  were  obtained  : 

Material. 

Precipitated  Ferric  Phos 

Precipitated  Ferric  Phos 

Dufrenite 

Dufrenite 

The  effect  of  lime  on  the  solubility  of  these  phosphates  was 
then  determined.  For  this  purpose  i  gram  of  each  of  the  ma- 
terials was  placed  in  a  flask,  pure  slaked  lime  (free  from  phos- 
phoric acid)  added,  and  enough  water  to  make  a  thin  paste. 
After  twenty-four  hours  the  contents  of  the  flask  were  treated 
with  100  c.c.  of  I  per  cent  citric  acid  solution  and  an  additional 
amount  of  the  crystallized  acid  to  neutralize  the  excess  of  lime. 
The  solubility  in  the  citric  acid  was  then  determined  as  in  the 
first  instance.     The  following  table  contains  the  results. 


P2O5  sol.  in 

Per  cent  of 

r  cent  citric  acid. 

total. 

12.20% 

31.9 

12.05% 

31-5 

0.07% 

0.4 

0.09% 

0.5 

cent  citric  acid. 

total. 

24.95% 

65.3 

24.90% 

65.2 

35.80% 

93.7 

35.80% 

93-8 

37.00% 

96.9 

36.90% 

96.6 

0.45% 

2.8 

0.47% 

2.9 

26 

Material.  I,ime.  P2O5  sol.  in  i  per      Per  cent  of 

Prec.  Ferric  Phos i    gram 

Prec.  Ferric  Phos i    gram 

Prec.  Ferric  Phos 0.5  gram 

Prec.  Ferric  Phos 0.5  gram 

Prec,  Ferric  Phos 0.25  gram 

Prec.  Ferric  Phos 0.25  gram 

Dufrenite i    gram 

Dufrenite i    gram 

The  following  table  shows  the  comparative  solubility  of  the 
phosphoric  acid  (PgOJ  in  i  per  cent  citric  acid,  with  and  without 
the  addition  of  lime  : 

Material.  Without  lime.  With  lime. 

I  gr-  0.5  gr.  0.25  gr. 

Prec.  Ferric  Phos.  __  12.20%  24.95%  35.80%  37.00 

Prec.  Ferric  Phos. __  12.05%  24.90%  35-90%  36.90 

Dufrenite 0.07%  0.45%              

Dufrenite 0.09%  0.47%              


It  is  seen  from  the  above  tables  that  the  action  of  lime  on 
the  natural  ferric  phosphate  was  not  very  marked.  In  the  case 
of  the  precipitated  ferric  phosphate  the  lime  caused  a  considerable 
increase  in  the  solubility.  The  larger  quantities  did  not  cause 
as  great  an  increase  as  the  smaller  quantities.  This  fact  indi- 
cated the  formation  of  the  more  soluble  calcium  phosphates  with 
the  smaller  quantities.  With  an  excess  of  lime,  as  in  the  experi- 
ment in  which  i  gram  was  used,  tri-calcium  phosphate  was 
probably  formed,  and  this  compound  is  much  less  soluble  in  i 
per  cent  citric  acid  than  the  di-  and  mono-calcium  compounds. 

It  is  to  be  remarked  that  the  reaction  was  fairly  rapid. 
Within  one  hour  after  the  addition  of  lime,  a  red  compound  began 
to  separate.  This  compound  was  probably  ferric  hydroxide.  An 
attempt  was  made  to  separate  the  products  formed  in  the  reaction. 
Various  solvents  were  tried,  but  it  was  found  impossible  to  effect 
a  separation  of  calcium  and  ferric  phosphates  in  the  presence  of 
ferric  hydroxide.  In  the  course  of  these  trials  a  5  per  cent  solu- 
tion of  neutral  potassium  oxalate  was  observed  to  act  on  precipi- 
tated ferric  phosphate,  dissolving  both  the  iron  and  the  phos- 


27 

phoric  acid  in  the  proportion  in  which  they  existed  in  the 
original  compound.  This  action  of  potassium  oxalate  was  made 
the  basis  of  a  qualitative  test  for  the  presence  of  ferric  phosphate, 
in  the  presence  of  ferric  hydroxide.  The  solution  was  found  to 
have  no  solvent  action  on  ferric  hydroxide.  The  compound 
formed  with  ferric  phosphate  appeared  to  be  a  double  oxalate, 
very  soluble  in  water. 

Ferric  phosphate  was  treated  with  varying  quantities  of 
lime.  The  products  of  the  reaction  were  treated  with  the  solu- 
tion of  potassium  oxalate,  filtered,  and  the  filtrate  tested  for 
iron.  The  absence  of  iron  in  the  oxalate  solution  was  taken  as 
showing  the  absence  of  ferric  phosphate  in  the  mixture  treated 
with  this  solvent.  0.2  gram  lime  was  found  to  be  the  smallest 
amount  capable  of  completely  converting  i  gram  of  ferric  phos- 
phate. 

The  behavior  of  potassium  oxalate  suggested  the  use  of  this 
reagent  for  determining  the  available  phosphoric  acid  in  phos- 
phatic  materials.  A  few  preliminary  trials  on  its  solvent  action 
on  calcium  and  iron  phosphates,  under  different  conditions,  were 
made.  The  results  were  not  sufiiciently  satisfactory  to  indicate 
the  usefulness  of  this  reagent. 

(b)      ACTION  OF  LIME  ON  THE  PHOSPHATE  FORMED  BY  FIXATION. 

It  is  fairly  well  established  that  soluble  phosphates  when 
applied  to  soils  tend  to  pass  into  comparatively  insoluble  forms. 
This  reversion  is  caused  largely  by  hydrated  oxide  of  iron.  It 
was  considered  important  therefore,  to  know  the  effect  of  lime  on 
the  compound  thus  formed. 

To  this  end  two  grams  of  pure  ferric  hydroxide  were  treated 
in  a  small  flask  with  50  c.c.  of  an  aqueous  solution  of  superphos- 
phate, shaken,  and  allowed  to  stand  ten  days  with  occasional 
agitation.  The  insoluble  residue  was  then  filtered  off  and  washed 
with  cold  water  until  phosphoric  acid  ceased  to  be  given  up. 
The  filtrate  and  washings  were  then  analyzed  and  the  phos- 
phoric acid  absorbed  calculated.  Several  residues  were  obtained 
in  this  way.  Two  of  these  were  treated  with  100  c.c.  of  i  per 
cent    citric   acid  solution,  and  the  solubility  determined  as  de- 


28 

scribed  under  ferric  phosphate.  Four  others  were  treated  with 
0.5  gram  of  lime,  enough  water  added  to  make  a  thin  paste,  and 
the  mixture  allowed  to  stand  one  and  nine  days.  The  solubility 
in  I  per  cent  citric  acid  was  determined,  sufficient  crystallized 
citric  acid  being  added  to  correct  the  basicity  due  to  lime.  The 
superphosphate  solution  originally  added  contained  0.1340  gram 
of  P2O5.  Under  "residues"  in  the  following  table  are  included 
the  amounts  of  PgOg  absorbed  : 

P2O5  in                                I,ime  added.  P2O5  sol.  in  Per  Cent 

residues.                                                                      i  per  cent  citric  acid.  soluble. 

0.1249  grs.  0.0214  grs.  17. 1 

0.1248  grs.  0.0224  grs.  17  9 

0.1244  grs.  o.  5  grs.  one  day  0.0544  grs.  43.7 

0.1240  grs.  0.5  grs.  one  day  0.0535  grs.  43.0 

0.1244  grs.  0.5  grs.  nine  days  0.0499  grs.  40.1 

0.1244  grs.  0.5  grs.  nine  days  0.0495  grs.  39.7 

Incidentally  a  similar  experiment  was  made  using  neatral 
ammonium  citrate  solution  (1.09  sp.gr.)  as  the  solvent.  For 
various  reasons  this  solvent  was  not  used  in  the  soil  investiga- 
tions. The  preceding  plan  was  followed  except  that  the  action 
of  lime  was  allowed  to  continue  for  two  days.  The  results  were 
as  follows  : 

P2O5  in  I^ime  added.  P2O5  sol.  in  Per  Cent 

residues.  am.  citrate.  soluble. 

0.1217  grs.  0.0535  grs.  43.9 

0.1211  grs.  0.0534  grs.  44.0 

0.1223  grs.  0.5  grs.  two  days  0.0536  grs.  43.8 

0.1224  grs.  0.5  grs.  two  days  0.0527  grs.  43.0 

The  results  show  that  lime  materially  increased  the  solubility 
of  reverted  phosphate  of  iron  in  i  per  cent  citric  acid.  The  action 
appeared  to  be  complete  in  one  day.  The  solubility  of  the  re- 
verted phosphate  in  ammonium  citrate  solution  was  not  increased 
by  the  addition  of  lime. 

The  composition  of  this  phosphate  of  iron  has  not  been  deter- 
mined. It  is  much  more  insoluble  in  i  per  cent  citric  acid,  and  am- 
monium citrate  than  the  normal  ferric  phosphate.  That  none  of 
the  latter  compound  was  formed  in  the  fixation  experiment,  was 
indicated  by  the  following  tests  : 


29 

( I ) .  Ferric  hydroxide  and  a  solution  containing  phosphoric 
acid  in  large  excess  of  the  amount  necessary  to  form  the  normal 
salt,  were  allowed  to  react  for  weeks.  The  amount  of  phosphoric 
acid  absorbed  was  then  determined. 

Fe(0H)3  used 0.2500  grs. 

P2O5  absorbed 0.0275     " 

P2O5  required  to  form  FePO^ 0.1664     " 

Even  in  the  presence  of  a  large  excess  of  phosphoric  acid, 
the  ferric  hydroxide  absorbed  little  more  than  one  tenth  of  the 
amount  necessary  to  form  the  normal  salt. 

(2).  The  compound  formed  was  treated  with  the  5  per 
cent  solution  of  potassium  oxalate  already  described.  After 
filtering  the  filtrate  was  tested  for  iron,  the  absence  of  which 
showed  the  absence  of  normal  ferric  orthophosphate. 

The  compound  formed  by  fixation  is  evidently  a  highly  basic 
phosphate  of  iron.  The  compound  appears  to  be  formed  even 
when  only  a  limited  amount  of  the  base  is  present.  In  the  soil, 
the  bases  would  usually  be  present  in  large  excess  with  respect 
to  the  phosphoric  acid. 

In  the  experiments  just  described,  the  lime  was  brought  into 
intimate  contact  with  the  phosphate  of  iron.  Its  action  there- 
fore was  probably  at  a  maximum.  It  was  thought  advisable  to 
supplement  this  with  artificial  soil  experiments  under  conditions 
which  usually  obtain  in  soils,  conditions  presumably  less  favora- 
ble to  the  action  of  lime. 

For  this  purpose  200  gram  portions  of  clean  sand  were  ex- 
tracted with  hydrochloric  acid  and  washed  free  from  acid  with 
cold  water.  A  blank  experiment  showed  that  the  product  pos- 
sessed no  absorptive  properties  for  phosphoric  acid.  Each  por- 
tion was  mixed  with  4  grams  of  ferric  hydroxide  and  placed  in  a 
six  inch  cylinder.  Superphosphate  solution  was  then  added  and 
absorption  allowed  to  continue  for  ten  days.  During  the  mean- 
time the  mixture  was  kept  moderately  moist.  After  ten  days 
the  phosphoric  acid  unabsorbed  was  washed  out  with  cold  water 
and  determined,  the  amount  absorbed  being  calculated  from  the 
data  thus  obtained.  The  mixture  then  represented  a  soil  con- 
taining 1.5  per  cent  Fe.Pg,  and  0.12  per  cent  of  P2O5. 


30 

For  this  work  and  the  soil  investigation  to  be  described  later 
two  solvents  were  selected,  namely,  i  per  cent  citric  acid  solu- 
tion, and hydrochloric  acid.  The  first  has  already  been  re- 
ferred to  ,  but  for  soil  work  the  conditions  attending  its  use  were 
modified  somewhat. 

Hydrochloric  acid was  recently  proposed  by  Moore^  for 

the  determination  of  available  mineral  plant  food  in  soils.  In  the 
work  referred  to  pot  experiments  were  made  with  a  large  variety 
of  soils,  and  the  amounts  of  phosphoric  acid  extracted  by  the 
solvent  in  question  agreed  closely  in  nearly  all  cases  with  the 
actual  amount  removed  by  the  crops. 

The  solubility  determinations  with  the  reagents  selected 
were  carried  out  in  the  following  manner.  The  ratio  of  solvent  to 
substance  was  500  c.c.  to  100  grams.  These  were  placed  in  a 
litre  glass  stoppered  bottle,  and  kept  at  a  temperature  of  40  de- 
grees C  in  a  water  bath  for  exactly  five  hours.  The  bottles  were 
shaken  every  fifteen  minutes.  After  the  digestion  the  whole  was 
shaken  and  emptied  on  to  a  folded  filter  sufficiently  large  to  hold 
the  entire  contents  of  the  bottle.  After  draining,  400  c.c.  of  the 
filtrate  representing  80  grams  of  the  soil,  were  evaporated  to 
dryness.  In  the  case  of  the  citric  acid  the  residue  was  treated 
with  magnesium  nitrate  solution,  evaporated  to  dryness,  and 
ignited  until  all  organic  matter  was  destroyed.  These  residues 
were  taken  up  with  water  and  nitric  acid  and  the  phosphoric 
acid  determined. 

In  all  the  experiments  a  preliminary  digestion  was  made 
with  20  grams  of  soil  and  100  c.c.  of  the  solvent,  in  order  to 
determine  the  basicity  of  the  soil.  A  correction  was  then  made 
in  the  strength  of  the  acids  so  as  to  reduce  the  solvent  action  to 
a  uniform  basis. 

Portions  of  the  artificial  soil  described  were  air-dried  and  the 
solubility  of  the  phosphoric  acid  determined.  Lime  (one  and 
two  grams)  was  mixed  with  other  portions  and  the  action  allowed 
to  go  on  for  seven  days,  the  mixture  being  kept  moist  during  the 
meantime.  These  mixtures  were  then  air-dried  and  solubility 
determinations  made.     The  following  results  were  obtained. 

I  Jour.  Am.  Chem.  Soc,  1902,  79. 


31 


SOLVENT — I    PER    CENT   CITRIC   ACID   SOLUTION 

P2O5  in  I^ime  added.  P2O5  sol.  in  Percent 

residues.  i  per  cent  citric  acid.  soluble. 

0-2530  grs.  0.0622  grs.  24.5 

0.2540  grs.  0.061 1  grs.  24.0 

0.2488  grs.  I  gram 0.1060  grs.  42.6 

0.2483  grs.  I  gram 0.1070  grs.  43.0 

0.2527  grs.  2  grams 0.1232  grs.  48.7 

0.2517  grs.  2  grams 0.1220  grs.  48.4 

SOLVENT — HYDROCHLORIC    ACID   

200 

P2O5  in  Lime  added.  P2O5  sol.  in  Per  Cent 

residue.  i  per  cent  citric  acid.        soluble. 

0.2530  grs.  0.0033  grs.  1.3 

0.2540  grs. 0.0038  grs.  1.4 

0.2488  grs.  I  gram 0.0044  S^^-  i-7 

0.2483  grs.  I  gram 0.0052  grs.  2.0 

0.2527  grs.  2  grams 0.0048  grs.  1.8 

0.2517  grs.  2  grams 0.0052  grs.  2.0 

From  the  results  it  is  seen  that  lime  increased  the  solubility 
in  I  per  cent  citric  acid  considerably  as  in  the  preceding  experi- 
ments. The  increase  is  somewhat  less,  however,  with  the  arti- 
ficial soil. 

With   regard  to  the  solubility  in hydrochloric  acid,  the 

addition  of  lime  produced  little  change.  The  differences  are 
within  the  range  of  experimental  error.  It  is  to  be  remarked 
that  the  solvent  action  of  the  hydrochloric  acid  was  very  small. 

(c)      ACTION   OF   LIME   ON   SOILS. 

For  the  purpose  of  studying  the  effect  of  lime  on  the  phos- 
phoric acid  originally  present  in  soils,  two  samples  of  the  latter 
were  taken.  The  soil  designated  "A"  was  a  sandy  loam  under 
cultivation.     Soil  "B"  was  a  clay  loam  not  under  cultivation. 

Samples  were  prepared  for  analysis  according  to  the  methods 
of  the  Association  of  Official  Agricultural  Chemists\ 

I    U.  S.  Dept.  Agr„  Div.  Chem.,  Bui.  No.  46,  71. 


32 

The  determination  of  total  phosphoric  acid  was  made,,  the 
method  being  to  weigh  out  2  grams  of  soil  into  a  platinum  dish 
and  ignite  to  drive  off  organic  matter.  Put  in  i  or  2  c.c.  of 
hydrofluoric  acid  and  allow  the  soil  to  come  in  contact  with  the 
acid  slowly  to  avoid  loss  by  sputtering.  After  the  violent  action 
has  ceased,  place  on  a  water  bath  and  evaporate  to  dryness.  After 
repeating  this  operation  once  or  twice,  take  up  with  a  little  nitric 
acid  and  water  and  determine  phosphoric  acid.  This  method 
was  rapid  and  gave  closely  agreeing  results.  In  this  way  the 
following  results  were  obtained  : 

Soil  "A."  Soil  "B." 

Total  F,0, o.  145%  o.  155% 

Complete  analyses  of  the  acid  soluble  materials  in  these  were 
then  made  according  to  the  methods  of  the  Association  of  Official 
Agricultural  Chemists\  with  the  following  results  : 

DIGESTION    IN    HYDROCHI^ORIC    ACID 

Constituent. 

Insoluble  matter 

Potash  (K2O) 

Soda  (NaaO ) 

Lime  (CaO) 

Magnesia  ( MgO) 

Oxide  of  Manganese  (MnO) 

Ferric  Oxide  (FcgOg) 

Alumina  (AI2O3 ) 

Phosphoric  Acid  (P2O5) 

Sulphuric  Acid  (SO3) 

Carbonic  Acid  (CO2) 

Volatile  matter 

Portions  of  200  grams  of  the  air  dried  soils  were  then  mixed 
thoroughly  with  lime  (one  and  two  grams),  the  whole  placed  in 
cylinders,  moistened  and  allowed  to  stand  exposed  to  the  atmos- 
phere for  seven  days.  The  portions  were  then  again  air  dried, 
and  weighed.     The  solubility  of  the  original  and  limed  portions 

in  I  per  cent  citric,  and  hydrochloric  acids  was  then  deter- 

200 


ID    I. 115    SP. 

GR. 

Soil  "A." 

Soil  "B.' 

87-85% 

83-53% 

0.74% 

0.80% 

0.20% 

0.41% 

0.11% 

0.14% 

0.55% 

0.78% 

0.03% 

0.02% 

1.74% 

1.91% 

4.00% 

5-72% 

0.12% 

0.10% 

0.04% 

0.06% 

0.26% 

0.18% 

4.17% 

6.51% 

U.  S.  Dept.  Agr.  Div.  Chem.,  Bui.  No.  46,  72. 


33 

mined  according  to  the  method  described  under  artificial  soils, 
the  quantities  taken  being  calculated  on  a  basis  of  the  original 
air  dry  soil.  The  quantities  of  phosphoric  acid  in  the  following 
tables  are  calculated  to  this  basis,  and  are  therefore  comparable 
in  all  cases. 

SOIL    "a."    solvent— I    PER   CENT   CITRIC   ACID 


Litne  added. 

None 

None 

One  gram 

One  gram 

Two  grams 

Two  grams 


;05  soluble. 

Per  cent  of 

total. 

0.028% 

19.3 

0.030% 

20.7 

0.031% 

21.3 

0.032% 

22.0 

0.037% 

25.5 

0.038% 

26.2 

SOIL      B. 


SOLVENT — I    PER   CENT   CITRIC   ACID 


Lime  added.  P2O.5  soluble.        Per  cent  of 

total. 

None 0.0068%  .4.3 

None 0.0067%  4-3 

One  gram 0.0132%  8.5 

One  gram 0.0130%  8.3 

Two  grams 0.0100%  6.4 

Two  grams 0.0102%  6.5 

SOIL   "A."       SOLVENT HYDROCHLORIC   ACID 

200 

Lime  added.  P2O5  soluble.  Per  cent  of 

total. 

None 0.0030%  2.0 

None 0.0030%  2.0 

One  gram 0.0043%  2.9 

One  gram 0.0041%  2.8 

Two  grams 0.0040%  2.8 

Two  grams 0.0038%  2.7 

SOIL       B."       SOLVENT HYDROCHLORIC   ACID 

200 

The  amount  of  phosphoric  acid  was  too  small  to  be  deter- 
mined. The  addition  of  lime  (one  and  two  grams),  seemed  to 
produce  no  effect  on  the  solubility  of  the  phosphoric  in  this  soil. 


34 

The  two  soils  used  in  the  experiments  described  contained 
practically  equal  amounts  of  total  phosphoric  acid,  and  phosphoric 
acid  soluble  in  hydrochloric  acid  1.115  sp.  gr.,  while  the  avail- 
bility  of  that  in  the  cultivated  soil,  as  shown  by  the  weak  acid 
solvents,  was  about  five  times  as  great  as  that  in  the  uncultivated 
soil.  The  chemical  analyses  of  the  two  present  no  differences 
that  could  serve  as  an  explanation  of  the  difference  in  availability. 
This  difference,  therefore,  is  probably  due,  partly  to  the  state  of 
cultivation,  and  partly  to  the  physical  characteristics  of  the  soils. 

From  the  results  obtained  it  is  doubtful  whether hydro- 

200 

chloric  acid  would  prove  a  suitable  solvent  for  the  determination 

of  available  plant  food  in  all  soils.     Comparing  the  two  soils, 

however,  the  results  with  it  indicate  in  a  general  way  the  same 

relative  availability  of  the  phosphoric  acid  as  is  shown  by  the 

solubility  in  i  per  cent  citric  acid. 

In  regard  to  the  action  of  lime,  the  solubility  in  citric  acid 
was  increased  slightly  in  both  soils.  The  solubility  in  hydro- 
chloriQ  acid  was  not  increased  in  the  case  of  the  uncultivated 
soil.  This  soil  evidently  held  the  phosphoric  acid  in  a  very 
insoluble  form. 

It  is  interesting  to  note  the  difference  in  action  of  lime  on 
natural  and  artificial  soils.  A  comparison  is  made  in  the  follow- 
ing table. 

SOLUBILITY   OF   PjOg   IN    I    PER    CENT   CITRIC   ACID 

Without  lime.  With  lime. 

Soil  ''A" 19.3%  21.3% 

Soil  "A" 20.7%  22.0% 

Artificial  soil 17.1%  43-7% 

Artificial  soil i7-9%  43-o% 

The  figures  represent  per  cent  of  the  total  phosphoric  acid . 
It  is  evident  that  the  natural  soil  was  less  susceptible  to  the 
action  of  lime  than  the  artificial  product. 

(d)       FIXATION   OF   PHOSPHORIC   ACID   IN    THE   PRESENCE   OF 
COMPOUNDS   OF   IRON   AND   CALCIUM 


While  the  tendency  of  soluble  phosphates,  applied  to  soils 
is  to  pass  into  the  insoluble  phosphate  of  iron,  several  investi 


35 

gators,  notably  Deherain,  KostitchefF,  and  others  have  observed 
that  calcium  carbonate  reacts  with  ferric  phosphate  forming  cal- 
cium phosphate,  the  action  taking  place  even  in  the  presence  of 
an  excess  of  ferric  hydroxide.  Whether  this  was  the  reverted  or 
the  normal  ferric  phosphate  was  not  stated. 

It  occurred  to  the  author  that  the  presence  of  a  relatively 
large  quantity  of  calcium  compounds  might  interfere  with  the 
formation  of  reverted  phosphate  of  iron.  A  short  experiment 
was  made  in  order  to  ascertain  whether  the  presence  of  calcium 
carbonate  would  influence  the  fixation  of  phosphoric  acid  by 
ferric  hydroxide. 

Tri-calcium  phosphat^  was  dissolved  in  water  charged  with 
carbon  dioxide,  and  200  c.c.  of  the  solution  containing  0.0176 
gram  of  PjOg  were  added  to  i  gram  of  ferric  hydroxide,  with  and 
without  varying  quantities  of  calcium  carbonate.  The  action  was 
allowed  to  continue  for  ten  days,  after  which  the  mixtures  were 
filtered.  The  solubility  of  the  residues  in  ammonium  citrate  solu- 
tion was  determined  by  adding  50  c.c.  and  allowing  to  stand 
twenty-four  hours  in  the  cold.  This  residue  was  filtered  and 
washed  with  cold  water.  The  results  are  contained  in  the  fol- 
lowing table  : 

Material  mixed  with  0.0176  grs.  P2O5.  P2O5  insoluble  in 

ammonium  citrate. 

I  gram  CaCOg 0.0018  grs. 

I  gram  Fe(  OH)  ^ 0.0145  grs. 

I  gram  Fe(0H)3  plus  i  gram  CaCOa 0.0152  grs. 

I  gram  Fe(0H)3  plus  2  grams  CaCOg 0.0147  grs. 

In  the  blank  determination  above  practically  all  the  phos- 
phoric acid  was  extracted  by  ammonium  citrate.  The  presence 
of  calcium  carbonate  (one  and  two  grams)  apparently  had  no 
effect  on  the  solubility  of  the  compound  formed  by  the  reversion 
of  phosphoric  acid. 

III.     Behavior  of  Phosphoric  Acid  towards  Humus 

That  humus  possesses  the  power  of  absorbing  phosphoric 
acid  from  solution  has  not  been  proven.  Organic  soils  have  been 
known  to  effect  solution  of  insoluble  phosphates  in  some  cases, 


36 

while  in  other  cases  soils  containing  a  large  amount  of  humus  ex- 
hibited a  strong  attraction  for  phosphoric  acid. 

In  studying  the  retentive  power  of  humus  for  phosphoric 
acid  it  was  thought  best  to  work  with  the  artificial  preparation, 
thereby  eliminating  the  possibility  of  the  interference  of  any  other 
absorbing  material. 

To  this  end  humic  acid  was  prepared  from  sugar  according 
to  the  method  described  by  Berthelot\  600  grams  of  sugar  fur- 
nished 100  grams  of  the  air  dry  material.  In  one  portion  the 
moisture  was  determined  by  drying  to  constant  weight  at  100  de- 
grees C.  Other  portions  were  subjected  to  combustion  analysis. 
The  following  analysis  is  calculated  on  the  basis  of  dry  matter. 

Carbon 61.48% 

Hydrogen 4.74% 

Oxygen 33-78% 

The  phosphoric  acid  to  be  absorbed  was  applied  in  the  form 
of  an  aqueous  extract  of  superphosphate,  thus  approximating 
conditions  which  obtain  in  actual  practice.  Portions  of  i  gram 
of  the  air  dry  humic  acid  were  placed  in  a  small  flask,  50  c.c.  of 
a  solution  containing  0.1550  gram  PgOg  and  0.0185  gram  calcium 
added  to  each,  and  the  mixtures  allowed  to  stand  one,  four  and 
eight  days  with  occasional  shaking.  At  the  end  of  these  periods 
the  insoluble  matter  was  filtered  off  and  washed  with  cold  water 
until  the  wash  water  gave  no  test  for  phosphoric  acid.  The 
quantities  of  phosphoric  acid  and  calcium  in  the  filtrate  and 
washings  were  then  determined.  Calcium  was  determined  by 
the  volumetric  permanganate  method.  The  results  are  found  in 
the  following  table. 

No.                  Duration  of  contact.  P2O5  soluble.  Ca  soluble. 

1  one   day o.i540grs.  0.0173  grs. 

2  one   day 0.1545  grs.  0.0180  grs. 

3  four  days 0.1535  grs.  0.0185  grs. 

4  four  days o.i550grs.  0.0188  grs. 

5  eight  days 0.1550  grs.  0.0180  grs. 

6  eight  days 0.1550  grs.  0.0177  grs. 

As  is  shown  by  the  above  results,  neither  the  phosphoric 
acid  nor  the  calcium  were  retained  by  the  insoluble  humic  acid. 

I  Chim.  Veg.  Agr.,  4,  123. 


37 

It  was  not  expected  that  chemical  absorption  would  take  place. 
The  experiment  was  suggested  by  the  statements  of  many 
writers  to  the  effect  that  humus  is  active  in  retaining  plant  food 
by  virtue  of  its  physical  structure.  The  data  given  above  indicate 
that  insoluble  free  humic  acid  exerts  no  influence  in  rendering 
phosphoric  acid  insoluble  in  water. 

The  organic  acids  present  in  the  soil  do  not,  however,  exist 
to  any  great  extent  in  the  free  state  except  in  bogs  and  morasses. 
Soils  suitable  for  agricultural  purposes  contain  little  or  no  free 
acid  except  carbonic  acid.  In  most  cases  they  give  an  alkaline 
reaction.  Humic  acid  exists  as  humates  of  the  bases  predomi- 
nating in  the  soil.  These  compounds  may  be  divided  into  two 
classes,  namely,  (i)  those  insoluble  or  but  slightly  affected  by 
water,  including  humates  of  calcium,  magnesium,  iron,  and 
aluminum,  and  (2)  those  soluble  in  water  including  humates  of 
the  alkalies.  Representative  of  the  first  class,  calcium  humate 
was  selected  for  study. 

This  compound  was  prepared  by  mixing  10  grams  of  the 
humic  acid  already  described  with  one  litre  of  lime  water  con- 
taining 2.42  grams  of  CaO,  and  allowing  the  mixture  to  stand 
for  five  days  with  occasional  shaking.  The  insoluble  residue 
was  then  filtered  off  and  washed  with  cold  water  until  the  wash- 
ings were  neutral.  On  being  subjected  to  combustion  analysis 
the  product  showed  the  following  composition,  calculated  on  the 
basis  of  dry  matter. 

Carbon 55-48% 

Hydrogen 4. 1 1  % 

Oxygen 34.8i% 

Calcium 5.60% 

This  preparation  was  treated  with  a  solution  of  superphos- 
phate in  the  manner  already  described  for  humic  acid.  The  solu- 
tion contained, 

P2O5 o.  1550  grs. 

Ca 0.0185  grs. 

The  quantities  of  phosphoric  acid  remaining  unabsorbed  by  i 
gram  of  the  calcium  humate  are  given  below. 


38 

Duration  of  contact.  P2O5  soluble.  Ca  soluble,  P2O 5  absorbed. 

One  day o.  I385grs.  0.0309  grs.  0.0165  grs. 

One  day 0.1385  grs.  0.0312  grs.  0.0165  grs. 

Four  days 0.1360  grs.  0.0278  grs.  0.0190  grs. 

Four  days o.  1365  grs.  0.0283  grs.  0.0185  grs. 

Eight   days 0.1340  grs.  0.0259  grs.  0.0210  grs. 

Eight  days 0.1345  grs.  0.0260  grs.  0.0205  grs. 

The  above  figures  show  that  absorption  of  phosphoric  acid 
took  place  to  a  limited  extent.  It  occurred  to  the  author  that  a 
small  amount  of  calcium  carbonate  might  have  been  formed  in  the 
preparation  of  the  calcium  humate,  in  which  case  the  absorption 
could  not  be  ascribed  to  the  latter  compound.  Carbon  dioxide 
was  determined  in  a  portion  of  the  preparation  therefore,  the 
amount  obtained  being  0.64  per  cent.  Calculated  to  calcium 
carbonate  the  one  gram  of  the  material  contained  0.0142  gram  of 
the  material  contained  0.0142  gram  of  the  carbonate,  which  in 
turn  would  be  sufficient  to  react  with  0.0207  gram  of  PgOg  form- 
ing the  mono-calcium  salt.  This  salt  is  decomposed  by  water  with 
the  formation  of  the  di-calcium  phosphate  which  is  not  soluble  in 
water.  The  absorption  therefore  is  to  be  referred  to  the  presence 
of  calcium  carbonate. 

The  increase  of  the  calcium  in  solution  was  due  to  the  for- 
mation of  some  of  the  mono-calcium  phosphate  from  the  free 
phosphoric  acid  present  in  the  superphosphate  solution. 

It  is  to  be  inferred  from  these  facts  that  calcium  humate  does 
not  take  part  in  the  absorption  of  phosphoric  acid  from  super- 
phosphate. 

As  the  majority  of  the  humus  compounds  found  in  the  soil 
contain  nitrogen,  it  was  thought  advisable  to  include  some  of 
these  in  the  present  work.  The  compounds  used  were  soluble 
and  insoluble  ammonium  humate. 

For  the  preparation  of  these  25  grams  of  humic  acid  were 
treated  with  500  c.c.  or  ammonia  solution  containing  14  3  grams 
of  ammonia,  the  flask  closed,  shaken  and  allowed  to  stand  five 
days.  The  insoluble  residue  was  then  filtered  off  and  washed 
until  free  from  ammonia.  When  dried  the  product  weighed  1 1 
grams  showing  that  a  little  more  than  half  of  the  humic  acid  had 
been  dissolved  by  the  ammonia.     The  soluble  portion  was  put 


39 

into  an  evaporator  and  allowed  to  stand  in  the  cold  until  all  free 
ammonia  had  evaporated.  The  solution  was  reserved  for  further 
investigation. 

The  insoluble  portion  was  subjected  to  a  combustion  analy- 
sis. The  "nitrogen  as  ammonia"  represents  the  amount  of 
nitrogen  expelled  by  boiling  with  magnesia. 

Carbon 62.29% 

Hydrogen 5-62% 

Oxygen 28.81% 

Nitrogen  (total) 3-28% 

Nitrogen  (as  ammonia) 1.08% 

Although  humus  varies  in  its  content  of  nitrogen  (depend- 
ing on  the  material  from  which  it  is  formed)  the  above  agrees 
fairly  well  with  the  average  analysis  of  humus  extracted  from 
soil  by  various  solvents.  Regarding  the  form  in  which  nitrogen 
is  held  in  this  compound,  Berthelot  has  shown  that  besides 
ammonia,  it  is  present  most  probably  as  amide.  This  fact  is  in 
accordance  with  the  data  obtained  when  working  with  the  nat- 
ural product. 

The  action  of  this  substance  on  superphosphate  was  investi- 
gated. The  material  was  treated  with  a  solution  of  superphos- 
phote  in  the  manner  already  described  for  humic  acid.  The 
solution  contained, 

P2O5 o.  1550  grs. 

Ca i. 0.0185  grs. 

The  quantities  of  phosphoric  acid  remaining  unabsorbed  by 
I  gram  of  the  insoluble  ammonium  humate  are  given  below. 

No.  Duration  of  contact.  P2O 5  soluble.  Ca  soluble. 

1  one  day 0.1540  grs.  0.0185  grs. 

2  one  day 0.1545  grs.  0.0178  grs. 

3  four  days 0.1540  grs.  0.0191  grs. 

4  four  days 0.1540  grs.  0.0183  grs. 

5  eight  days 0.1540  grs.  0.0178  grs. 

6  eight  days 0.1540  grs.  0.0187  grs. 

The  results  show  that  the  insoluble  ammonium  humate  ex- 
erted no  influence  in  retaining  the  phosphoric  acid  of  superphos- 
phate.    The  content  in  calcium  was  also  not  affected. 


40 

The  soluble  ammonium  humate  already  referred  to  contained, 

Nitrogen  (total) 20.25% 

Nitrogen  (as  ammonia) 9-65,% 

calculated  on  a  basis  of  dry  matter.  With  a  solution  of  super- 
phosphate, this  substance  produced  a  brown  flocculent  precipitate. 
Thinking  that  this  precipitate  consisted  of  calcium  humate, 
another  portion  of  the  ammonium  humate  solution  was  treated 
with  lime  water.  No  precipitate  was  formed.  Neither  calcium 
chloride  or  ferric  chloride  produced  any  change.  On  adding 
pure  orthophosphoric  acid  a  brown  precipitate  was  formed  appar- 
ently identical  with  that  formed  with  the  superphosphate  solu- 
tion. These  facts  indicated  the  formation  of  an  organic  com- 
pound containing  phosphoric  acid.  An  attempt  was  made  to 
separate  this  compound,  but  it  was  exceedingly  difficult  to  wash, 
and  apparently  underwent  partial  decomposition. 

From  the  foregoing  data  it  appears  that  insoluble  humus  offers 
no  resistance  to  the  extraction  of  the  phosphoric  acid  of  super- 
phosphate by  water.  The  soluble  humus  appears  on  the  other 
hand  to  form  an  insoluble  compound  with  phosphoric  acid,  and 
thus  acts  as  an  absorbent.  To  what  extent  such  absorption  may 
take  place  was  not  determined. 

(b)      SOLUTION 

The  solvent  action  of  two  of  these  humus  compounds  was 
considered  briefly.  For  this  purpose  precipitated  tri-calcium 
phosphate,  humic  acid,  and  soluble  ammonium  humate  were 
selected.     The  experiments  were  carried  out  as  follows. 

Portions  of  0.5  gram  of  pure  precipitated  tri-calcium  phos- 
phate were  placed  in  small  flasks,  and  to  some  of  these  was  added 
I  gram  of  insoluble  humic  acid,  and  10  c.c.  of  water.  To  other 
portions  were  added  10  c.c.  of  the  solution  of  ammonium  humate. 
The  contents  of  the  flasks  were  shaken  occasionally  for  five  days, 
after  which  the  residues  were  filtered  off,  washed  with  cold 
water,  and  the  phosphoric  acid  determined  in  the  filtrate  and 
washings. 

A  blank  experiment  was  made  with  0.5  gram  of  precipitated 
tri-calcium  phosphate  and  10  c.c.  of  water  treated  as  described 


41 

for  the  other  experiments.     The  results  of  these  tests  are  given 
in  the  following  table. 

Material.  P2O5  soluble  in  water. 

Tri-calcium  phos,  0.5   gr none 

Tri-calcium  phos.  0.5  gr none 

Tri-calcium  phos.  0.5  gr.  am.  humate  10  c.c._  none 

Tri-calcium  phos,  0.5  gr.  am.  humate  ioc.c._  none 

Tri-calcium  phos.  0.5  gr.  humic  acid  i  gr 0.0030  gr. 

Tri-calcium  phos.  0.5  gr.  humic  acid  i  gr. 0.0028  gr. 

The  ammonium  humate  was  without  action  on  the  tri- 
calcium  phosphate.  The  free  humic  acid  rendered  a  portion  of 
the  phosphoric  acid  soluble  in  water,  though  the  action  was  not 
very  great.  These  facts  are  in  accordance  with  the  observations 
of  other  workers,  namely,  that  humus  is  active  in  rendering 
insoluble  phosphoric  acid  available  to  plants.  The  results  pre- 
sented herewith  indicate  that  this  action  is  due  to  the  humic 
acid  and  not  to  the  compounds  of  humic  acid,  namely,  the 
humates. 

IV.     Absorption  of  Phosphoric  Acid  by  Zeolites 

There  is  abundant  evidence  to  show  the  existence  in  soils  of 
easily  decomposable  silicates  analagous  to  the  zeolites.  The 
power  of  these  substances  to  fix  the  bases  potassium,  sodium, 
calcium,  ammonia,  and  magnesium  has  been  thoroughly  investi- 
gated by  a  number  of  workers. 

The  work  of  Doelter  (i),  Friedel  (2),  Rinne  (3),  Clarke  (4), 
and  others  on  the  constitution  and  properties  of  the  zeolites,  sug- 
gested to  the  writer  the  possibility  of  the  power  of  these  sub- 
stances to  absorb  phosphoric  acid. 

For  this  investigation  both  the  artificial  and  natural  silicates 
were  used.  The  first  was  prepared  by  treating  a  solution  of  soda 
alum  with  a  solution  of  sodium  silicate.  The  resulting  precipi- 
tate was  washed  thorougly  with  cold  water,  filtered  and  dried. 
The  product  was  difficult  to  wash,  and  seemed  to  undergo  partial 
decomposition  even  in  cold  water,  with  the  elimination  of  sodium 
silicate.     The  dry  material  was  found  to  contain  the  following  : 


42 

Alumina i8. 12% 

Silica 57-21% 

Soda 5-85% 

Water 18.82% 

Two  grams  of  this  substance  were  treated  with  superphos- 
phate solution,  allowed  to  stand  seven  days,  filtered,  washed,  and 
the  phosphoric  acid  in  solution  then  determined.  The  residues 
were  treated  with  100  c.c.  of  i  per  cent  citric  acid  solution  and 
allowed  to  stand  for  twenty-four  hours.  The  phosphoric  acid 
dissolved  by  this  reagent  was  then  determined,  with  following 
results  : 

No.  P2O 5  absorbed.        Percentsol.ini 

per  cent  citric  acid, 

1  0.1340  grs.  56.8 

2  0.1340  grs.  57.4 

The  natural  silicates  used  were  selected  with  a  view  of  ob- 
taining as  much  variety  in  composition  as  possible.  A  few  insol- 
uble silicates  were  also  used. 

It  is  not  within  the  province  of  this  paper  to  enter  into  a 
discussion  of  the  constitution  of  these  compounds.  The  empir- 
ical formulas  are  given  herewith. 

Analcite  ^ NaAlSizOg       H2O 

Chabazite        (CaNag  )Al2Si40i2         6H2O 

Halloysite       H4Al2Si209        aq. 

Heulandite     H4CaAl2(Si03)6  3H2O 

Prehnite         H2Ca2Al2(Si04)3 

Pyrophyllite H2Al2(Si03)4 

In  order  to  determine  the  absorptive  power  of  these  sub- 
stances for  phosphoric  acid,  2  gram  portions  of  each  were  placed 
in  small  flasks,  and  to  each  were  added  50  c.c.  of  a  solution  of 
superphosphate  containing  o.  1550  gram  P2O5.  The  experiment 
was  carried  out  in  the  manner  described  for  the  precipitated  sil- 
icate. Solubility  determinations  of  the  residues,  in  i  per  cent 
citric  acid  were  made  in  a  similar  way.  A  check  determination 
was  made  with  washed  sand.  The  following  table  contains  the 
data  obtained. 


43 

Material.  P2O5  absorbed.      Per  cent  sol.  in  i 

per  cent  citric  acid. 

Sand none 

Analcite 0.0305  grs.  46.5 

Analcite 0.0295  grs.  44.7 

Chabazite 0.0170  grs.  loo.o 

Chabazite 0.0165  grs.  lOo.o 

Halloysite /    0.0125  grs.  80.8 

Halloysite 0.0125  grs.  80.8 

Heulandite 0.0130  grs.  loo.o 

Heulandite 0.0125  grs.  loo.o 

Prehnite 0.0125  grs.                       77.6 

Prehnite 0.0120  grs.                      76.6 

Pyrophyllite none  

The  results  show  that  some  of  the  natural  silicates  (zeolites) 
possess  an  absorbtive  power  for  phosphoric  acid,  while  the  arti- 
ficial preparation  did.  so  to  a  marked  degree.  The  compounds 
formed  appeared  to  be  more  soluble  in  i  per  cent  citric  acid  than 
the  compound  formed  when  ferric  hydroxide  was  the  absorbent. 

The  nature  of  this  absorption  is  not  understood.  In  the  light 
of  our  present  knowledge  of  the  zeolites  present  in  the  soil  it  was 
considered  impracticable  to  undertake  to  ascertain  its  nature. 

It  occurred  to  the  writer  that  this  absorption  might  be  due 
to  the  presence  of  free  hydrated  oxide  of  aluminum,  in  which  case 
the  action  could  not  properly  be  referred  to  the  silicates  in  ques- 
tion. In  order  to  determine  this  point,  some  representative  sam- 
ples of  the  materials  used  were  strongly  dehydrated  over  the 
blast  lamp  and  the  absorptive  power  again  determined  with  the 
following  results  : 

Material.  Before  dehydration  After  dehydration. 

P2O5  absorbed. 

Analcite 0.0305  grs.  0.0348  grs. 

Analcite 0.0295  grs.  0.0348  grs. 

Chabazite 0.0170  grs.  0.0170  grs. 

Chabazite 0.0165  grs.  0.0175  grs. 

Precipitated  siHcate o.  1340  grs.  o.  1 170  grs. 

Precipitated  silicate o.  1340  grs.  o.  1 170  grs. 

With  analcite  dehydration  seemed  to  increase  the  absorptive 
power  though  only  to  a  slight  extent.     Dehydration  of  the  arti- 


44 

ficial  preparation  decreased  slightly  the  absorptive  power,  indi- 
cating the  presence  of  a  small  amount  of  aluminum  hydroxide. 

It  is  doubtful  whether  there  exists  in  soils  silicates  identical 
either  with  the  natural  or  the  artificial  materials  used  in  the 
above  work.  But  it  is  believed  that  the  data  here  presented 
serves  to  show  that  the  zeolitic  silicates  in  soils  may  take  part 
in  the  fixation  of  phosphoric  acid. 

V.     Action  of  Ferrous  Sulphate 

The  value  of  ferrous  sulphate  in  agriculture  appears  to  be 
questionable.  Regarding  its  action  towards  phosphoric  acid, 
two  possibilities  suggested  themselves,  namely,  (i)  the  hydrol- 
ysis and  consequent  solvent  action  of  the  sulphuric  acid  formed, 
on  the  insoluble  phosphates,  (^2)  the  action  in  retaining  the 
phosphoric  acid  applied  in  a  comparatively  soluble  form. 

The  solvent  effect  on  several  phosphatic  materials  was  deter- 
mined by  placing  i  gram  of  each  in  a  flask  and  treating  with 
I  gram  of  ferrous  sulphate,  100  c.c.  of  water,  and  shaking 
occasionally  for  four  days.  The  residues  were  filtered,  washed, 
and  the  filtrate  tested  for  phosphoric  acid.  None  was  present  in 
any  case.  The  residues  were  then  treated  with  100  c.c.  of  i  per 
cent  citric  acid,  and  the  solubility  of  the  phosphoric  acid  in  this 
reagent  determined.  The  following  table  contains  the  data 
showing  the  effect  of  ferrous  sulphate  on  the  solubility  in  i  per 
cent  citric  acid. 

SOLUBILITY    IN    I    PER    CENT   CITRIC    ACID 

Material. 

Tri-calcium  phosphate 

Tri-calcium  phosphate 

Ferric  phosphate 

Ferric  phosphate 

Apatite 

Apatite 

Wavellite 

Wavellite 


No  FeS04. 

FeS04 

O5  dissolved. 

P2O5  dissolved. 

26.40% 

20.40% 

26.20% 

20.50% 

12.05% 

ir.50% 

12.20% 

11.25% 

1.47% 

1.36% 

1.47% 

1.44% 

trace 

trace 

trace 

trace 

45 


Ferrous  sulphate  did  not  increase  the  citric  acid  solubility  of 
the  phosphates  under  investigation.  In  fact  with  the  tri-calcium 
the  effect  was  to  lessen  the  solubility. 

The  remarkable  effect  of  ferrous  sulphate  when  used  with 
superphosphate  as  was  the  case  in  the  field  tests  made  by  Grif- 
fiths, indicated  that  the  substance  converted  the  phosphoric  acid 
into  forms  more  available  than  the  reverted  phosphate  formed 
from  ferric  hydroxide. 

The  precipitate  formed  on  the  addition  of  superphosphate  to 
ferrous  sulphate  was  found  to  be  easily  soluble  in  i  per  cent  citric 
acid.  In  order  to  determine  whether  this  compound  is  formed  in 
the  presence  of  ferric  hydroxide,  2  grams  of  the  latter  were 
mixed  with  0.5  gram  of  ferrous  sulphate  and  50  c.c.  of  super- 
phosphate solution.  Of  the  reverted  phosphate  of  iron  formed 
from  ferric  hydroxide  without  the  sulphate,  17.3  per  cent  of  the 
phosphoric  acid  was  soluble  in  i  per  cent  citric  acid,  while  the 
compound  formed  when  ferrous  sulphate  was  present  showed 
19.0  per  cent  citric  acid  solubility. 

It  is  improbable  therefore  that  ferrous  sulphate  can  be  of 
any  benefit  as  a  retentive  agent  for  phosphoric  acid,  especially 
when  there  is  sufficient  ferric  hydroxide  present. 


46 


CONCLUSIONS 


Summarizing  briefly,  the  writer  thinks  that  the  following 
conclusions  are  supported  by  the  foregoing  work. 

I .  Lime  increases  considerably  the  solubility  of  precipitated 
ferric  phosphate  in  i  per  cent  citric  acid  solution.  The  nature 
of  the  calcium  phosphate  formed  depends  on  the  relative  amount 
of  lime  added.  The  reaction  takes  place  with  moderate  rapidity. 
The  solubility  of  dufrenite  is  increased  by  the  addition  of  lime, 
though  not  to  a  very  marked  degree. 

II.  Reverted  phosphate  of  iron,  formed  when  phosphoric 
acid  is  absorbed  by  ferric  hydroxide,  is  not  identical  with  normal 
ferric  phosphate.  .  It  is  much  less  soluble  in  i  per  cent  citric 
acid.  The  addition  of  lime  to  reverted  phosphate  of  lime  in- 
creases its  solubility  in   i  per  cent  citric  acid.     The  solubility  in 

hydrochloric  acid  is  increased  only  to  a  slight  extent.    The 

solubility  in  neutral  ammonium  citrate  solution  is  not  increased. 
Reverted  phosphate  of  iron  appears  to  be  a  highly  basic  compound, 

III.  Lime  applied  in  moderate  amounts  to  the  soils  used, 
increased  the  solubility  of  the  phosphoric  acid,  in  i  per  cent  citric 

acid,    though   the   increase  was   small.       The  solubilitv  in  — r— 

200 

hydrochloric  acid  was  affected  to  a  less  degree,  and  in  one  of  the 

solis  used  no  action  occurred.     The  latter  reagent,  as  a  solvent 

for  the  available  phosphoric  acid  in  soils,  is  of  doubtful  utilit)^ 

IV.  The  phosphoric  acid  originally  present  in  soils  is 
much  less  susceptible  to  the  action  of  chemical  agents  than  re- 
verted phosphate  of  iron.  The  latter  probably  becomes  more 
resistant  with  age. 


47 

V.  The  presence  of  calcium  carbonate  in  relatively  large 
amounts,  appears  to  have  no  effect  on  the  solubility,  in  am- 
monium citrate,  of  the  compound  formed  by  fixation. 

VI.  Insoluble  humic  acid,  calcium  humate,  and  insoluble 
ammonium  humate  offer  no  resistance  to  the  extraction  of  the 
phosphoric  acid  of  superphosphate  by  water.  Soluble  ammonium 
humate  forms  with  superphosphate  solution,  an  insoluble  com- 
pound whose  composition  and  properties  have  not  been  deter- 
mined. The  absorptive  property  of  humus  is  due  probably  to 
the  humus  compounds  soluble  in  water. 

VII.  Soluble  ammonium  humate  does  not  effect  solution  of 
precipitated  tri-calcium  phosphate.  Insoluble  humic  acid  renders 
a  portion  of  the  phosphoric  acid  soluble  in  water. 

VIII.  Some  of  the  zeolites  and  similar  hydrous  double  sil- 
icates are  capable  of  abstracting  phosphoric  acid  from  solution. 
The  property  is  not  destroyed  by  strong  ignition.  The  com- 
pounds formed  are  comparatively  soluble  in  i  per  cent  citric  acid. 

IX.  The  solubility  of  tri-calcium  phosphate,  ferric  phos- 
phate, apatite,  and  wavellite,  in  water  and  in  i  per  cent  citric 
acid,  was  not  increased  by  the  addition  of  ferrous  sulphate. 
With  tri-calcium  phosphate,  the  solubility  in  i  per  cent  citric  acid 
was  decreased,  indicating  that  the  application  of  ferrous  sul- 
phate under  some  circumstances  may  be  detrimental. 

X.  Ferrous  sulphate  forms  with  superphosphate  a  compound 
soluble  in  i  per  cent  citric  acid.  In  the  presence  of  ferrous  sul- 
phate, and  ferric  hydroxide,  the  compound  formed  with  super- 
phosphate, has  practically  the  same  solubility  in  i  per  cent  citric 
acid  as  the  compound  formed  when  ferrous  sulphate  is  absent. 
The  latter  possesses  therefore  no  especial  value  as  a  retentive 
agent  for  phosphorics  acid. 


^ 


> 


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